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.)