Tested it using a drill and it was completely seized, so I stripped it down as far as I could go to try and fix the problem - turns out it's jammed/stuck between the bit that spins & accepts the drive and the housing (if that makes sense), but I can't strip it down any further to try and fix it, as it's held by 2 rivets that I don't fancy drilling out as I don't know how I could refix it.
The jammed speedo broke the drive cable too

So the question is: has anyone got any experience with this problem and any ideas on how to fix it without drilling the rivets?
Or if not, what speedo could I replace mine with? The serial number is: 4419/04 1376 in a 1965 minor 1000, although I'm thinking this is perhaps not the right one? I'm not particularly concerned with originality, so do I just need to find one with a similar tpm to 1376?
Thanks everyone!