I have used the "green stuff" for this job (I THINK it is Prestone), but any of the antifreeze solutions should work just as well.

Disassemble the engine as far as your abilities allow (minimum - remove carb, backplate and head), place in the crockpot, cover with antifreeze (full strength) and put on low overnite (or longer).

DO THIS OUTSIDE !!! The fumes ARE nasty.
DO NOT USE THIS CROCKPOT FOR FOOD AGAIN !!! The antifreeze gets into the pores of the crockpot and will poison any food cooked in it.

You MAY need a toothbrush to scrub some of of the stubborn spots (inbetween the cooling fins, muffler ).
The engine will come out looking like new.

Wash with soap and water, then rinse well in fresh water and put in a 200 degree oven for about 30 minutes to completely dry the parts. Lubricate the engine with after run oil and reassemble.