Come on! A little JB Weld and that block is good as new.
That is an interesting design - the cylinders are bolted onto the crankcase, just like a Porsche.
If it can't be fixed, I would bet if you started searching the "links" section and talking to some of the crustier old stovebolters, I'll bet someone has a replacement block sitting out back in the shop.
As an alternative, that is an extremely simple design. No oil passages, just a water jacket and cylinder sleeves. You might find some guys who could actually sand cast a replacement block. Not cheap, but doable in the worst case.
-Boyo
