I did it once with a totally froze up engine, wouldn't turn over at all. I boiled it in anti-freeze in a coffee can out back on the side burner of my gas bar-b-q.
In no time it freed the engine up & within an hour or so the thing looked like it was brand new on the outside as well. Never occurred to me what the anti-freeze would do to the rubber seals as Ken mentioned.[&:] Guess I need to replace them all now before I try to use that engine.[:@]

I believe Piper Chuck has another cleaner method using alcohol & the oven. I'll let him explain it as it is his method & I don't want to mis-quote him.