I replaced the radiator on my S1 vn about 18-24 months ago, I went to my local natrad and got one for under $200 (can't remember exactly, but it wasn't what I'd call expensive)
Honestly it wasn't hard to find at all, and I didn't even think about doing any s2 radiator conversion as from what I know there's a bit of work involved, I still probably wouldn't even bother..
I'd also suggest replacing your thermostat and maybe your water pump while you're there too, I noticed a very big difference after changing the radiator, thermostat, water pump as well as all hoses, wasn't worried about temperature for months (until I lost all of my coolant, probably because I'm an idiot, and well.. water doesn't seem too cool as well as coolant, plus I haven't got around to putting the bottle I have, into the car..

)
Good luck either way! and when you replace the radiator I suggest giving the system a fairly good flush, I went the hack on mine for 30-60 minutes and it didn't get dirty again for a long time, I feel with a good flush along with correct maintenance every so often a good system will go far. (also, I was told by the local radiator joint not to put the 'radiator flush' stuff through my system, I had the cash out prepared to buy it and the bloke selling me coolant told me not to bother, and just to be very thorough with water.)