I have an unused silver century plus longshaft. Is it wrong to use it do you think? I haven't even run it yet. Has original 8com plug etc. etc. Believe it or not it was an ebay purchase, for £100 before the classic boats article!
Bet you it's the water pipe. Had a similar problem myself. Thought just the channels in the head blocked, after usual stripping de-rusting etc, turned out the pipe was blocked and detached from the base of the engine block