Can't help you with the muffler removal, but you can use several different solvents to clean your engine.

Acetone, mineral spirits, kerosene,  or even model airplane fuel should work.

I'd remove the back plate and head, soak the engine and those parts in the solvent for a few minutes, and then swish the engine around to flush out anything that's loosened. Do this in a glass jar with a tightly fitting lid, and watch for crud to settle to the bottom. Repeat with clean solvent, until no more crud settles.

When your satisfied that no more curd is being loosened, drain the engine and parts on a paper towel or clean rag, and then blow it out with canned air, or blasts of air from a tire pump, or rubber squeeze bulb. Be sure to wear safety glasses. Lube the engine inside and out when finished.

Let us know how it goes.

Bill