OK, I think from the replies just about any kind of liquid will work. I'm thinking the toilet for a couple days and keeping using as normal might even work. The agitation and acid might even help. Can't be worse than the dishwasher or coke.

Seriously, there might be some seels in there that would be ruined by some of these processes. Most likely there's something in there that isn't metal. The engine runs on glow fuel so I'd guess that would be the safest to soak it in.