I just tried putting a display of an AW3 GPS onto an AW3 LTE. I even tried moving the original NFC module of the LTE version onto the GPS’s display and ended up with pairing issues. Was able to solve the pairing issues by using the display + NFC module from GPS version but had to sacrifice support for Apple Pay. There is already a discussion about this topic here: Why won’t my watch pair after screen replacement?
"Or can i buy a digitizer for AW2 and use it to replace the one broken on the AW3 GPS version" This should work fine. There isn’t any difference in the digitizer from AW2 vs AW3 - also between GPS and LTE.