The only way to know for sure is to pull it apart and inspect everything unless you find the plug or head is loose. If the cylinder and piston are in good condition and wear is within limits, then you can get away with rings and gaskets. If you lost power that quickly, it's not going to be just normal wear. :( Also, you may not hear noise when a problem exists. My top and bottom were trashed yet the bike was quiet and still had good compression. Therefore, inspection is the best way.
Marc -
