RE: another darn ryobi post (ryobi performance modification, analysis and comparison)
About those exhaust holes.... Can you drill them bigger and re-tap? Next idea would be brass inserts or helicoil. Last choice would be having them welded shut and re-drilling and tapping. The reason I say "last choice" is because of the possiblity of warping the whole cylinder when welding it, though that might not happen....
AV8TOR