Chances are these engines were all run with a castor oil mix in the fuel. While I have no objections to the use of Caster Oil (I always have some in my fuel) it can cause the engines to gum up over time. A prolonged soak in kerosene or alcohol will help to disolve this but you may need the addition of heat. Soak in an old electic crockpot overnight (on the lowest setting and outside as the fumes are poison) filled with the old type antifreeze. This will disolve the old varnish on both the inside (you did take off the backplate and take out the glow plug first?) and outside of the engine. It may take a bit of scrubing with an old toothbrush to get it all off. I have also heard that carburator cleaner works well also but have never tried it. The reason I say "old type antifreeze" is that you want the type with glycol as a base, I believe the newer type has a different chemical composition due to the poison factor. Never ever reuse a crockpot that has had antifreeze in it for food preperation as the poison will still be there.