Try to start with good parts. These things are too cheap to find to be hassling with less than perfect parts going into the project.

The Ryobis run good, and are nice for a narrow cowled airplane like a Tiger Moth biplane or a Mustang. But if you are after high power and rpms, then go with the Homelite 30cc or find a 30cc Echo. (The Homelite 33cc engines out of chain saws run well too.) The Stihl 25.4cc engine is another good one in this size range. If you can find an Echo 23.6cc engine, they are very lightweight and make very good power, especially when modified. I have one that turns an APC 16 x 8 at 9000 rpms. (It is hopped up...) They came in PB-2400 blowers and other Echo machines in the mid to late '90's.

The Ryobi is power limited by the stock stamped steel rod. If you run them over 7500 rpms for very long, they'll blow....

AV8TOR