Hey Rusyan,
I think an engine like the st61 would work fine if it weren't for that pesky muffler ruining a scale look... I'd try to find something like that but maybe in a rear exhaust version (MVVS .60?) and use either a pitts muffler or pipe (I'm pretty sure you could hide it in a custom-made bellypan). If it were me, I'd stick with either a 4c or a gas engine.... but that might not look scale!! --just some Ideas!
jilayu udachi