I hate to be depressing, but down this road lies a lot of grief because those studs go into water jackets and leaks are not always obvious. Water can migrate into the manifold and cause all kinds of slow acting destruction. This in the end was what ruined one of my engines
So....the real repair requires the engine out of the boat and at a machine shop.
The *I need to fix this now and cheap* repair may involve the manifold. If you are SURE the stud is in good and water-tight, I would be very tempted to drill out the manifold just enough to get it on there.