Gabriel, sorry to be the messenger for the bad news, but you can probably forget about that idea: Reason being that these MMCx000 carts were pretty tiny and very light. Hence I'd deem it hardly possible to find another cartridge the originals could be replaced with without running into all sorts of problems (mainly unsuitable height, so VTA/SRA are wrong, unsuitable weight, so the balange range of the counterweight might not suffice, unsuitable length, so ideal arm geometry can't be achieved and distortion will be a lot higher...). The only thing I could imagine remotely possible is that one might be able to create an adaptor for the older MMCx an MMCx0 carts - but as these usually aren't any cheaper or easier to dig up, creating such an adaptor would seem a lot of work for hardly any use.
Greetings from Munich!
Manfred / lini