TwoStroke, 95% of the SSC flyers use OSLA or Magnums .15s with good success. The Leo.15 is also a super engine. Any of them will do a good job.

Your problem is the plane. The Gnat will not be close to competitive in SSC combat. It's a fun plane to fly for sport or combat and easy to build and would be good for a one design club combat event, but it's just a target towing a streamer in open SSC.

Good luck

Cajun[8D]