Hi oregoncarl, yeah the engine start will kick-back, without any rotating mass like a blade or a heavy flywheel, soft aluminum flywheel key will often become sheared during engine testing when you have no rotating mass, the vibration of the engine while testing will be excessively intense…rotating mass fixes that. However once you get the alternator/pulleys/belt bolted up to the engine crank the alternator itself will give you a good amount of rotating mass, quite probably enough that the kick-back & vibration will not be as noticeable. The belt tension with the alternator must be adjusted fairly tight, any slippage in the driveline rotating mass will be seen as starter kickback/excessive vibration/broken flywheel keys…
If it is true the crankshaft is too long in your application, you can permanently alter the thing by cutting-off the excess length, but if you go in that direction, I would make sure to redrill the hole[blade bolt hole] much deeper to the depth of about 4.5" before you cut-off the excess length, in this way the [blade bolt hole] hole will remain after you whack off the excess length…as you might need that straight bolt hole to help hold the pulley on there. At best that engine model is a 5 HP on gasoline, as it is only a 12 cu. in. displacement, so additional rotating weight could be a very helpful thing as far as torque & engine smooth running goes. Maybe you can hang some extra rotating mass on that 3 5/32" shaft, if it becomes practical.
As for the PCV Positive Crankcase Ventilation, that valve is generally used to keep a ‘partial vacuum’ within the engine crankcase. That partial vacuum inside the crankcase is important, because it helps keep the engine from consuming excessive oil, and it helps keep the crankshaft oil seals from leaking, that PCV valve keeps the vacuum inside the engine, and pulses of air pressure come out the discharge pipe. So then where to plumb the PCV to? You could plumb it into your homemade fuel/air intake manifold, or you could vent it to the atmosphere [not the best air emissions result], but think about plumbing it toward your wood or charcoal hopper of the gasifier itself. Plumbing directly to the gasifier fuel supply hopper should reduce the PCV pulsating effect upon your system, & keep the emissions of it, incinerated.
As for the ‘Blade Brake Compliance stopping device’, on the front of your engine, yes that can be safely removed, by simply removing it, but ONLY because your changing the application, you are no longer intending to mow grass with a blade, your intending a generator, of which the BBC laws do not require such a stopping device [as far as I know]. The device can remain tho, as a practical ON/OFF switch, the heavy spring may have to be replace to a lighter duty spring, where you can actually work the thing easily. I would recommend you have an working ignition switch of some kind…to solely rely upon fuel shut-off is OK for a diesel engine, but not so much on a spark ignited engine. If you don’t like the BBC device on there, a simple ON/OFF toggle switch will give you the ability to control the ignition.