I think you need a new engine. A guy at our field has the same issue with a Hobbico trainer and what we found is that because his carb got knocked off and was sucking air, the engine would lean in flight. After a while it got to the point that it was hard to run. What we did was remove the glow plug and turn the engine over by hand. It bound while turning it over without the glow plug installed. I would try that and see if you have any excessive wear or damage.