Hmmmmm.......And old OMC is going to be difficult to find parts for, I think... Now if it was an old Johnson
I know Zuback's in the Sayerville area used to carry all the old OMC stern drive stuff so you might want to try them.
If you do find a power head, most just come with the block and you have to bolt on all your old parts. Probably going to be super hard to find one with carb, power pack etc...
If you manage to find one and it's just the block, make sure you find out what made the old one blow in the first place, or the new one could suffer the same fate.
One other thing to consider.... If the parts are expensive to fix yours, you might be better off going with a used outboard that is more main stream brand. At least if it breaks you will be able to find parts for it.
Good luck. I hope it works out for you.