Mike, its an oldish engine in my (limited) experience when the powerhead on my Suzuki had to come off to change the engine mounts, places were quoting over £400. For a marine engineer to do the complete job on your outboard would cost similar, and being an old engine its really not worth spending alot on it.
I would replace it with a Mariner 40HP the older ones are rebadged Yamahas and bombproof engines. Me and a mate have had 2 one a 1980's model and the latest a 2001 which I have just sold. Heres a couple for sale:
Mariner 40HP £899
Mariner 40HP £500