I'm not sure I'd take it apart. You're going to the swap meet in less that two weeks. You may find a tach drive distributor there. Then you just get the generator and you're golden.
Or, you can buy a magneto. I can spin those in my machine. That would be cool.
That is similar to the tach I used for years. I found the inductive pickup was more accurate than convetional triggering from the coil. I used to send all my tach's somewhere back east that tested them. Most of them were off.
I'll be checking my new tach very close when I install it.
