Check with Twin City Outboard in Shokopee Mn. Sorry I don't have a phone number but they should have a web site, or Google the name. They usually have about everything, thing is they like to charge for it too. Either that or do a web search for marine or outboard salvage yards I know there's a bunch of em out their.
Just a suggestion-For a motor that old I would consider a complete re-power with either a new or newer used DFI outboard. I would guess you could find something like a 5 year old optimax for the same cost as what your repair bill will be. Check with a mercury dealer as well, I know for a fact mercury sells remanufactured power heads, I just dont know if they would still make one to fit your application. Good luck and hope you can get back on the water!