you can take the unit apart - I did it 3 times - by unfolding the crimped over edge. Inside is a diaphragm: cloth impregnated with rubber or similar that acts on the lever that screws in the middle - a spring holds it in its quiescient state.
It may be posibld to repair it - I failed so made one from some bicycle inner tube - unfortunately that almost worked but geting the crimped edgeto seal is difficult or so I found.
In the end I got one from Wadhams; seem to remember it being quite pricey; £40 or so. And sod's law meant the mounting on the new unit did not match my dizzy so I had to drill and tap the dizzy; apparently I had the rarer model.
You may find a later dizzy, RR or SD! going cheap but you would need to sawp the oil pump gear over; having said that, it is easy to conver from p6-<sd1 gears but not sure the other way; if not, you would use the oroiginal spindle; more messing but not that hard.