@SpongeBran
Swapping wheels can work, but it's not just about getting the tire to fit the rim, it's about the rim fitting the tractor's hub safely.
Make absolutely certain the bolt pattern, lug nut type (conical vs. spherical), and especially the load rating of the Kubota wheels are compatible with your Massey.
You don't want to overstress a wheel designed for a lighter tractor. If you're struggling to get precise measurements or specs, it might be worth finding a specialized tractor wheel and tire shop (not just a general tire shop) in your area.
They might have the cross reference info or be able to fabricate an adapter safely if needed