One alternative: take the engine to a "small engine" repair shop, where they are experienced in working with lawn mower, chain saw, and weed whacker engines.

Another alternative: I suggest you search the forums (use advanced search to select specific discussion forums) for anyone else who has experienced the same problems you are having. If you can't find anything which helps, start a new thread here describing your engine, and the problems you are experiencing. I'm sure someone will be able to help.