I took a few links of chain in to the farm supply place. It has been taken over by NAPA, but the old boy still has piles of his own stuff in the back room. He's normally pretty good at finding old stuff, but I think his eyes are not so good these days, as he had me measure the links, then phoned his supplier, who told him that this size was in his book, and sure enough it was!........so, I ordered a roll and it should be in tommorow.
Thanks for the input to everyone. I'll see how the chain fits on the original sprockets, as it isn't so easy pulling the sprockets off to get new ones.