I'm sorry if this is dumb, but could you simply use silicone caulk in a tube?

Kind of like how you'd use RTV or similar "gasket in a tube" in auto repair? Lay a bead around the boiler flange, wait a bit for the material to cure, then bolt down the boiler lid.

Not dumb at all. That was one of the options i considered and the one i was suggested by a plumber. A couple of people here said that it would be problematic to use silicone caulk and recommended silicone sheet instead. I don't know if it would work, haven't used the stuff before. If it works properly then that would make things a lot easier for future restorations.