Have you considered re-using the head gasket
Or is my neck to red for you! A propane torch and some gental heat will help anneal the metal ring surounding the cylinder and it "may" seal up again, at least only two of the four would have to seal, so your odds just doubled! I've never tried it, but have contemplated it when the funds were low. I have done it with copper head gaskets and it worked every time.
On the issue of counterbalance, consult with an engine shop that does balancing of rotating assemblies to determine how much wieght should be attached to the the crank in the position of the missing cylinders. I'd just use the old con rod and weld an attachent on that would be ok to float, ie. cylindrical.