Um, you didn’t hear this from me but if you were to open up a dowel (of the type you might find between the head & barrel or sidecover) & squeeze it back over the non moving mounting point under the brake-shoes you would eek out some more life for a bike you don’t want to throw a whole lot of money at.
Have heard some speciality shops can fit a new drum liner. You’d be lucky I would have thought, try classic repair places maybe?
Failing that I have done the rear brake conversion on a drum wheel. Fit a round piece of ally & carefully weld it to the drum flush where the backing plate normally slides into, bolt the donor disc to this (find a small one from a wrecker) then weld a mounting bracket for the calliper to the wheel spacer that also slots onto the swingarm mount for the drum so it doesn’t rotate. Careful to plan new seal mounts.
When welding to a hub loosen off all the spokes first & re-tighten when cool.
Have seen people ally weld mounts to the forks & bolt on a calliper & find a wheel to fit. Risky as could well distort the slider. Have to disassemble first of course.