It is possible that your engine is not toast yet, but if it is then you may just consider a replacement engine if your mechanical skills are few att his point, if you do ahve a good assortment of tools and a good place to work, you may want to tackle a rebuild but if you have to replace more than just rings, gaskets and the cylinder you can purchase a new slant head engine from BGF on ebay for less than the cost to completely go through the one you have now, it reall yjust depends on what is needed in the rebuild, if it has never completely siezed up on you then it's very possible that a basic top end rebuild could get you running good and strong again.
Heres links to the replacement engine alone I spoke of above 4 types Black slant head, Chrome slant Head and standard straight plug engine all good choices in my opinion:
Motorized BIKE GAS ENGINE - 80cc motor EPA black slant - eBay (item 190441041812 end time Oct-06-10 18:02:15 PDT)
Motorized BIKE GAS ENGINE - 80cc motor EPA slant Chrome - eBay (item 150486198351 end time Sep-27-10 13:22:17 PDT)
Motorized BIKE GAS ENGINE parts - 80cc motor EPA black - eBay (item 170378494424 end time Sep-26-10 17:27:46 PDT)
Motorized BIKE GAS ENGINE parts - 80cc motor silver EPA - eBay (item 170494273408 end time Sep-29-10 22:06:52 PDT)