http://www.bowersflybaby.com/tech/tach.html is quite a good read if thinking of a wire wrap round a plug lead - which is probably easiest IF it will work reliably.
That's an aero-site, but same applies except for the shielding on aircraft HT parts to eliminate radio interference. There's a link there to all sorts of instruments that might do, analog ones at that, but not sure how they are for 'cheap'.
Otherwise, I think it's Hall effect sensors and a bit of thought as to 'where to put', but above my pay grade. Easier, I think, with magnetos with external coils than with rotating ones for reasons to do with interference (most modern aircraft mags are static coil / magnetic rotor I'm pretty sure, although older ones used mags like ours).
My long-late mate KenF, who used to post on here years ago, successfully made one for his Vincent Rapide by making a disc, with black and white sections, that sat outboard the contact breaker on his KVF mag, on an extension of the centre screw that holds it on, with an optical sensor to pick up the rotation. He then made a matching 'chronometric' dial and casing which had a lot of people fooled as it looked really 'right'. It worked well for a while . . . but I fear I no longer have the diagrams, or specs of the bits he used. It was a long time ago.
