Worth trying a local auto electrics place for the starter. We were able to get starters over the last few years for MF50 digger (1976 Perkins 4.212) MF35 tractor (1960 Perkins 3.152) and for the boat's 1980 Volvo MD7a. The two Perkins were off-the-shelf Lucas.
Is the pump really unfixable? - bearing - £15, seals 2 x £3 from my Jabsco dealer in Glasgow. When replacing, get someone to turn the engine over on the starter (stop lever out) while you progressively tighten the adapter plate screws to allow it to centre properly. That way you don't need the special tool. a new pump is about £150.....