For that Tecumseh, it would be easy to build a charcoal gasifier.
For a lot of fun in the riding mower you could also build a charcoal gasifier.
But that requires burning and snuffing out and grinding and classifying the fuel. Not easy in the suburbs with easily annoyed neighbors.
You could build a Simple-Fire updraft charcoal gasifier, but I would change to a “flute” nozzle.
Updraft charcoal gasifiers save a lot on weight, a few have made gasifiers for their lawn tractors/mowers.
For engines that small, I would look at the charcoal gasifiers. There is a whole section on the site dedictated to the ‘dark side’ as we call it.
Kind of depends on what type of fuel you want to run, Rick. Charcoal or wood/pellets. I have run a generator that size with just a charcoal simple fire. For the riding mower, fitting it on the machine is a consideration. I’d lean toward one of Matt R’s ammo box designs. I don’t do wood so can’t comment on that.
Here’s some of the lawn tractors that people have put together.
Don Mannes:
Jeff’s “Air Carbon Fuel Cell”:
Now that we know you have the skills and equipment. Do you have metal break? Can it bend 16 ga steel? If so take a look at my ammo box tutorial. If anything It will show the basics of my cross draft design. If you can build a box you can make a more robust and larger version of this to run your tractor.
When you get to the blower filter and lid come get me or ask these guys. This is a dumbed down system meant for anyone to experiment with. For the filter you should build a simple drum filter and we can show you how to mount the Ametek blower to it.
This here bellow is the latest nozzle update. You dont need to purge the water tank with this new design.
The dimesions for a unit that will work accross all your engines should be.
10 to 14 inches wide
18 inches deep
30 inches tall.
Nozzle port hight is 6 inches above exhaust exit port on oposing side or the top of the grate. Its actauly a diagonal cross draft.
Use a minimum of 16 ga mild steel 14 ga would be best but depends on your break.
These machines are tollerant to some wood fuels mixed in the charcoal and less than engine grade charcoal. This is running at the higher end though. The smaller engines it will not be as tollerant.
For the lid, weld a flat flange on the top. Add a hinged lid and use flanged rope seal with a center clamp plate to secure the rope seal to the lid. The top flange plate needs to be very flate so no warping, I weld an angle bar structure under ours when we fab the commercial systems. Along with the sides of the hopper box. Then use the spring latches they used on the old Jeeps.
The “V” hearth should reach to the hight of the nozzle and you will want to suspend a grate about 2 to 4 inches above the bottom to allow the grate to screan out the charcoal and then you gas flow will exit from the grate and then out the exhaust exit port. You may want to weld or build a bolt on drop box to the back. This will allow some dust settling and give the gas some time to cool so you can manage the gas with out buring up hoses.
I missed that video somehow Don. I made my first charcoal gasifier from your tracker design. If I built a lawn tractor I’d copy this one as well. Kind of perfect.
Tom, I was under the impression everyone in the whole world with access to internet knew about Don’s lawn tractor wheelie skills 
matt, good work as always, i only miss your dog on the video…
Good Morning Rick,
Great that you have now defined out you for-sure intended usages for using wood as a basis for motor fuel.
The tiny, 5000 RPM 80CC inverter-generator will be the most difficult engine to satisfy.
So do make that one a small dedicated charcoal fueled system. And accept getting only 1000 watts.
The riding lawn mower to be still ride on movable must have a very lightweight unit too.
So make that one also a dedicated charcoal fueled until too. Will need to be capacity larger.
You third desired unit can be charcoal-portable like MattR shows up well.
Or a larger raw wood fueled unit like in this video:
Or this one:
I am somewhat alarmed that you could never even get a flare from that first build up.
You are either missing some ONE key element that will later bite you in the ass again. Like say . . . trying to use wet wood.
Or you had multiple small things stacking up on you which will again occur and set you back.
Like . . . too hard of system blowing/sucking; air leaking in; not truly getting up internally to good reactions temperatures; too wet of charcoals and/or woods put in. Etc.
Lessons you will have to learn going forward with anything you would build up.
Regards
Steve Unruh
No suburb issues here. I’m on a few acres in the sticks. 
Well then making Charcoal is no issue then for fuel.
This sir, is EXACTLY my worst fear. Common sense says I should have gotten a flare. I spent days going over every aspect figuring that I would discover some issue. But, not joy. I have decided to try one last light on the old unit with the filter box eliminated. I’m sure it’s air tight, but… gotta be some small thing killing it
Yep. I actually made a small charcoal maker from an old barbecue grill which works ok for small scale operations. Of course it didn’t help the ASSifier that I built. 
Hello Rick .
If you are producing smoke out the flare pipe something is not right . You should see a haze but not smoke. Maybe from this video you can judge the velocity of the bowers . This is a raw wood gasifier .
Why, there’s wind pumps (or generators?) in the background. I’ve never noticed those before. @Wayne, can you tell a little about those?
beyond the blower, you said you made char which is an excellent start, but do you have enough fuel in it to maintain the hot temps. And make sure the wood is dry. You may also need a variable speed controller for your blower motor.
If it is an air leak, you can pressure test it with soapy water like you do tires. Cover the holes and blow it up with the air compressor, apply soapy water and look for bubbles. It is NOT uncommon for pin prick size holes in welds that cause issues.
This video is a small version but it shows basically how the FEMA gasifier I started with was supposed to run with the blower forcing air in instead of sucking the gas out.
It looks like this one made smoke just like mine did but it should ignite. Notice that he adjusted the fan position to control the flame.
My current charcoal gasifier can be flared similar to this except I force air into the nozzle. It is difficult or maybe impossible to get a flare with the blower at full speed. Since I don’t have variable speed on the mattress inflater, I just hold the blower a little away from the nozzle which reduces the amount of gas coming out where I test for a flare.
I have never really gotten mine to flare by pulling the gas through the blower but I do use it that way to light the gasifier and sometimes to pull the gases through the hose to get it close to my engine.
I did the bubble check. Minor leaks which were addressed. No joy.
I filled the burn tube with charcoal and mounted a variable speed controller for the fan. I’ve tried every fan speed imaginable. No joy.