Flywheel, valve springs, air filter, exhaust, etc. Try a search above. Or, for the same or less cost, you can drop in a bike style motor and it will do better than the modded clone.
theres tons of parts and upgrades for them on ebay but you can save alot of money by making your own exhaust and intake, drill out the carb jet... or if you want to take it all apart get timing advance and billet rod, flywheel... basically every part but the block.
ill make a pipe for you if ya want, gotta decide how long though.
oh and if u need to weld anything i got a welder....