When I did the Farm All Tractor it was upgraded with a turbo. Once I had this tractor tuned I could not tell the difference between gasoline or the woodgas. Never had any issues during the time I had it. I recently talked with this customer and he sayes its still running fine. Whats interesting is this is set-up even the the carburetor it is running both the air and fuel mixture. So running petrol periodically to clean is easily done.
As far as tar and soots are concerned the high centrifugal forces with throw the heavy liquids off to the side keeping it clean. But having the ability to run liquid fuels for cleaning maintenance I think is something to consider.
Another thing to consider in any engine is a Cam swap. You want one with a longer duration and possibly a different overlap. Hopefully Steve or Peter could chime in on the over lap and give direction on this. A cam change will be the simplest and most effective for your investment.
We will be working on all these things later this year, developing a Vortec 5.3 engine. This engine will have electronic controlled spark ignition that we will have control over by the turn of a knob.
Here is a turbo I am looking into for our generators. http://www.ebay.com/itm/111189119062?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1438.l2649