The T6 class requires a stock G62, the other classes vary..
Unlimited class is just that, anything goes, engine must not weigh more than 14 lbs..We use Herbrandson 289cc twin drone engines, 9000 rpm,Aerrow 200 twins,10,000 rpm, Husky 8.8 and 11.6 twins, 10,000 rpm,ZDZ twins, and maybe a few others..All were derived from chainsaws except the ZDZ..The formula one and biplane classes have a limit of 80cc, so the engines used are Q75, Husky 4.4, Zenoah twins, and sometimes a Stihl or something else in that size..The engine of choice in this class is the Zenoah twin..No limit to these except 80cc...they turn around 11,500 rpm in the air...We tune these on the ground with a 15-8 prop to turn 11,000 and simulate the carb mixture conditions in the air....Props are all carbon fiber, fuel is methanol and nitro in varying percentages..We ran the Herbrandson twins with no nitro..
Note that there are no "modern" engines in the bunch, with the exception of the ZDZ...
Dave Johnson tried a 3W 120 about ten years ago, not successful...
There isn't enough money in racing engines to get a manufacturer to build a dedicated race engine, so we just get along with our old underpowered outdated chain saw conversions and go 230 mph The Herbrandson 289 makes around 32 hp with no tuned pipes..