Yes, you can usually get the radiator re-cored. I used this option on my Mk2. I did however note that the radiator was very similar to that found on M10 engined BMWs, specifically:
E21 316/318
E28 518/518i 1982-1987
E30 316/318/318i 1981-1988
I didn't actually get one but it was roughly the same size and the input/output points looked nearly identical. You can get them from
http://www.gsfcarparts.com or
http://www.eurocarparts.com and they're under £80 quid. No idea if it'll work, but from a dimensions point of view, it looks alright.
Stewart