Kostas, as an intermediate (& cheap) trick, remove the muffler baffle, & additionally, drill out the exhaust nipple with the next-size-larger drill. That is -- try drill bits up its arse until you get a close fit, then drill one size larger.

You will have to re-tune it, but it will gain a bit more power. It could be as much as 200-300 RPM if you are lucky. Your engine should also pick up a bit with a few more hr of run-time.