Which mk4 golf rear calipers for conversion??

Need some help!!!

I got a pair of rear mk4 calipers to swap out for my mk2 as they have been sticking.

I managed to snap a bleed nipple off one, couldn't get it out with an easy out or drilling it out.

So I've been told there are calipers with different size pistons.

The one I have left is 31mm approx at the end of the piston, I've asked a guy on eBay about one he is selling and that's 36mm.

So which model caliper so I need??

Anyone?