I got mine fitted with a new tripple core and bolted it back in the same hole.. no mess no fuss...

You could always get a quote to have a new core fitted.
I had a heavy duty core made with more tubes & fins than stock, but still with two rows of tubes.
Doing it this way has some advantages, like
1. The radiator is a known quantity.
2. It should come with a warranty.
3. You know it will fit the car without modification.
4. You don't need to modify the hose fittings.
5. You don't need to scrounge through hundreds of hoses to find something that works, as stock hoses fit.
6. It's bigger than a 1200 one, so you know it will do the job, even with a bigger engine.

Have a good look at your radiator core. If it is crumbling & coroded, then I can understand why they wouldn't provide a warranty. After close to forty years, it's probably well past its use-by date.

Whatever radiator you use, see if you can get a coolant filter. The one I use & recomend is like a cheesecloth condom & your local radiator specialist should know about them.

I think my coupe had a Corolla one in it

I have got the feeling that it depends upon the battery tray... are you going to leave it put?

The Gemini radiators are the best, but with the lightweight 1000s the stock radiators are usually okay.

Well the rad in my 1000 that i had cleaned and resealed has sprung a leak(about 3weeks after resealing). But they wouldnt warent it coz they recon its rooted! Pricks!

So what other cars rads will fit? Im going to the wreckers on sat for a look around but i would prefur a new rad!

Cheers!