If it were bearings, it would make all kinds of noise. I don't think that is it. Check the plug, maybe put a new one in it. Check your fuel, take the needle out and blow the carb out and make sure it is clean, then put the needle back in. Check to make sure you are not getting bubbles in the fuel from vibration, ballence the prop good.

Supertigre made good engines. I have run quite a few of them. they are not the strongest on the market, but they run great.