The gunk is very likely stop leak which Farmer Bob kept a putting in the radiator so he wouldn't have to sell a kid or two for enough money for a new brass radiator. VERY common occurrence in these old engines.
You probably won't find anything that will dissolve it. Mechanical means is the only thing that will break that stuff up to get it out.
It is a tedious enough job to accomplish with a bare block. With a bare block you can use a stiff steel rod down through little coolant holes in the top of the deck to reach quite a bit of it at the bottom water jacket. Water pump off and freeze plugs out helps quite a bit too.
