I don't know why you call it the wrong cartridge. You can use a Pioneer, Shure, Audio-Technica, Denon, Benz, Dynavector, or any other standard cartridge. If the 5MC was the original, perhaps it was ugraded to the 6MC.
Pioneer cartridges would be very hard to find, anyway.
If it sounds good, enjoy it.
BTW, the Alignment Protractors link at the top will give you the tools to make sure your cartridge is optimally positioned for the best sound.
Keep them spinning.