Hmmm... This could be a bit tricky. The OS-61FX and 65LA are VERY close in power output- I really like my Tower-75 not sure how it runs on a Pitts style muffler (I assume you want an in-cowl exhaust? I would suggest the Supertigre G90, it is inexpensive, runs well in different exhaust systems and is pretty powerful for its' size.

I am not familiar with the Evolution--

Other folks are going to ignore your budget comment and suggest 4 strokes I bet though...