There are 4 different radiator sizes that I know off.
430mm,525mm,630mm and 720mm. You can fit up to 630 on the MK1 easy. You might to use a smaller battery and relocate the power steering reservoir if you have any (probably not as you never got PAS in UK). The last two are utilising a dual fan setup with one motor and a belt to drive the second fan.
Other than that, any radiator will work just fine provided the system is clean and there are no leaks.