Merc last used a Ford engine way back in 1977. That makes that engine/drive AT LEAST 44 years old. Instead of trying to resurrect it, let it die gracefully, and re-power with something from THIS century.
As a dealer, we wouldn't touch anything more than about 20 years old. Too many 'down the track' problems. Example, you bring that in and ask for a carburetor clean out. I agree and do the job, charge you parts and about 3 hours labour (includes test running the engine in a test tank). A week (or a month, or 6 months) later you're back with 'it's not running right, you must have done something'. I investigate and find it's a failing fuel pump. Nothing I touched, was asked to touch, and had no reason to touch, but you think it was my fault, and in the eyes of the law it was, because I was the last to work on the engine... I now have to repair your failed fuel pump free of charge, and I get a bad rap from you to your mates. Bad reputations are very hard to erase or tell someone that it wasn't me, it was just that the customer (you) didn't understand that it was just one of those things. And as an engine gets older, 'those things' happen more and more, it's the second law of thermodynamics. Machines get old and more and more things fails on them, regardless of how much we try to stop that. It's just a fact of physics.
TL;DR, it's too old, update to a (much) newer model.
Chris........
Still runs but not any mechanics to work on it.