First verify it loses spark when it shuts down to 100% rule out a gas issue.
If it IS the mag, it's probably a failing coil which should be readily available and not REAL expensive and it's easy to replace. COULD simply be a thermally-sensitive condenser.
If you want to convert it to a distributor, PROBABLY the easiest to find is a setup off of a Farmall A/B/C etc..
As far as a diagram, Bob Melville has a series of excellent diagrams for Farmalls, there;s one that will work for your tractor