TOPIC:

will the mags from a 81 750 four fit a 76 750 twin?

They won't bolt on, so it depends on how much modification you want to do

750 twin 76-80 has 20 mm rear axle and 17 mm front axle, the 750 four has a 17 mm rear axle and 15 mm front axle.

77-80 KZ1000 wheels will bolt on a 750 twin, and the KZ650 front wheel will work, but not the rear.