I had a similar problem on a Mariner 75 (one of a pair).
1. Get an HT lead and try to strech it...not to much to break it though, and run your fingers down the length of it to see if any wires are poking out, if there is the leads have broke down. I have had the twice.
2. It could be that the magnets in the flywheel have broken down, get a finger agin and run it around the lower lip of the fly wheel and see if there are any black deposits that could be dust from the magnets.
3. Take to Alan Milton at the Outboard Shop in Tarbert Street Gourock, what he doesnt know about them is not worth knowing.
P.s If you break down again give me a call (my mobile is on the bottom of my e-mails). I can usually duke
away from work