I might look into the ST2300, the OS 120 is a bit on the high side.
Hi,
I personally think that these engines are a bit too much for the 90. The plane flies very well with an OS 91 FX, as you suggested, and a 15x8 or 14x10 APC. It is obvious that the plane should be built light. Moreover, a tuned pipe could definitely bring you the power you need, but the standard OS 91 muffler is quite good, too.
Regards,
-Fabrizio