OS number 8 or McCoy 59s are great plugs about 5 each.... if you ve never really got the motor running right, or well enough, then i would doubt your engine needs rebuilt unless it was run so lean the temps got really HOT over 350 hot......chances are its a small airleak in the fuel system or carb/engine area..... i would check for that first... no need rebuilding an engine thats not needing the rebuild.. pull your tank pressure hose from your exhaust pipe.. close your HSN fully. then blow into the pressure hose, look/listen for leaks in your lines adn tank, if all s good, open the HSN while blowing in tank and listen for air coming thru carb.. when you hear that, plug the carb opening with your finger getting it as airtight as you can, now blow again and look for leaks around the needles, crankcase ends, carb mount, head.ect. any air excaping except there the exhaust is bad and needs attention to get airtight... haveing an air leak will drive you nuts... runs great one min then dies the next at 1/2 thrt.. GRRRRRRRRRRR... im betting you ll find some air excapin somewhere it should not be..
hth
Timmahh