What a shame that those keycaps don't fit MX!
Is it because it's too small? I wonder if you could just shave the plastic on the inside of the mounts on the caps until its loose enough to fit MX.
MX stems are just the tiniest fraction too thick, but I think you could shave it slightly and it would fit. That's not the big issue however.
The problem is that MX switches rely on the keycap disappearing up to the stem into the switch, but this is impossible with these caps because they are virtually solid. Back in those days, there was no height requirement for switches yet, so it was not necessary to have this mechanism in place.
If they had fit, they would surely have been the ULTIMATE MX caps.