Should be able to get hold of a replacement sump for that engine I'd have thought - it was common to other Honda's of that era? Very likely only from Honda though, unlikely to get a decent one from a scrap yard (rust and if the engine's out of the car it'll have been sat on the sump which will have deformed it).
I'm sure an engineering company will be able to weld/braize a new boss in to the existing one though, that they can tap for a new thread. So long as it isn't excessivly corroded, it should be fine?
It's a nightmare when you buy an old car and keep finding bits wrong with it at first, isn't it. Keep heart though, I'm sure you can get through it and it'll be worth it in the end.
