I'm doing the same sort of thing to my Rowena engine (see other thread). But unlike yourself, I havn't thought it through Already bought ignition kit.

The magnet needed is tiny so can be glued into the 'hub'. As long as there is a bit of diameter on the hub (so it isn't super-critical on gap setup) I should be able to make it work. The hub itself is escaping me at the moment as I only have a plain ~12mm dia shaft on the front of my engine. So I'm looking for a plastic clamp/clip that I can drill for the magnet and clamp on the shaft.

If you've got a mag on yours, you should be able to put a nylon rod or similar on to the shaft to fit the magnet into. I've also read about hole-sawing the alloy core out of the flywheel and using that if you need to go onto a taper.

EDIT:- Found what I need......