ORIGINAL: piper_chuck
The GP-42 is toward the lower range of power for .45 size engines. It's ok for trainers and high wing slow planes. However, it won't be so useful if you eventually decide to move it to something faster.
I know that this isn't relevant to the trainer discussion, but just as a matter of interest, if you have ever seen a GP 42 on a Quickie, you'll change your mind in a hurry about how fast one of those engines will push a model.
It is actually better-suited to a speed model than a trainer. It makes an actual 1.11 dyno-tested HP at a genuine 16,500 RPM. Unlike many (most?) sport engines on the market, it actually reaches its advertised max HP, and at the advertised RPM. It will easily push a Quickie 500 past 100 MPH & I've clocked my clipped-wing Gremlin at 99 MPH with a TT 42 GP.