Search the forum. There has been a ton of discussion on this plane.

I owned two of these planes. I stuffed an OS50SX in it. I kept getting in trouble at the airfield because of noise. I tried both a bisson and slimline pitts. Neither of them would get the engine down under 100 db. I had to prop the plane such that I lost the screaming vertical.

I would go with a good four stroke in this airplane. A YS 63S would meet noise restrictions and give awsome performance.

Change the covering out on the bottom of the plane. Replace with bars. This plane gets small quickly and it is very easy to loose orientation on it.

Watch you speed with this plane. I had a top right aileron explode in flight due to flutter. Yes, the hinges were sealed. I believe it was do to very slight play in the interconnect between the bottom and top aileron.

Other modification I did included a carbon fibre landing gear and changing the position of the elevator servo in order to take differential out of the elevator.

Have fun with this one. The next ultimate I get will be bigger and easier to see.