You will be best off with a side mount on a 2C because ot the exhaust configuration (esp with the Pitts style muffler). I love the sound of a 2C engine on a pitts style muffler esp the big engines ... almost like a gas engine. The Ryan STA is a lovely plane and at the weight its coming in a 90 sized 2C is just about enough only. Nothing great, more for scale flying. Go with a 108 2C ... remember its about 11.5 lbs. Again, a 4C engine sounds kewl too for scale flying when you putter about.

Of course if you have the cash then go for a nice 4C like the YS110 or YS140DZ.