KD750 counterweight stub diameter, 13.7mm
KD650 .... 13.5mm, yup, it will not quite fit on.
Jelco ... 12mm
If you manage to get a whole KD650 arm, you can drop the whole thing in, keeping the height adjustment from the KD750. The arm from the 650 is a slightly better arm (better wiring mainly), but there isn't much in it. Alternatively, you could transplant the counterweight stub + counterweight from the 650 arm to the 750. I have a jelco sub and counterweight on the 750 arm bearing. It fits in just fine.