I soak old gunked engines in the "dregs" from the fuel cans. (that last bit that's hard to get with the fuel pump pickup, and is usually somewhat water contaminated anyway.)

The fuel will free up an engine that's been frozen due to dried/gummy fuel residue which comes from long term storage. Just pour some down the intake, and in via the glow plug hole, let the engine sit overnight, sealed in an old "Cool Whip" tub or similar. If its really bad... fill the tub to cover the engine.

You still might need to dissasemble the engine and clean out the rust and dirt... But everything will brush off easilly with an old toothbrush after soaking in fuel. Then dry the parts with a lint free rag. (a scrap T-shirt rag is good enough usually. After a bunch of washings... there's not much lint left in that material.)

I have used a can of nitro cleaner (from the hobby shop about $3.98) To clean the engine still on the plane..... Worked real good but let that motor dry and close the carbarator, That stuff is very flamable... so only use it on a cool motor---> learned the hard way on my rc car!!!

Cover the carb opening, and spend a couple minutes on the engine with a wire brush. For an engine that runs fine but is just gunky on the outside it's one of those "why didn't I try this years ago" type of things.