I purchased multiple alloy 50 mm ebay items ( sorry local suppliers/ welders , i just couldn't justify 4 or 5 times the price) approx $90.

you need to measure your widths, as the cheap ones vary from about 51mm to 58mm. ( also check visually on the fall of the height from top rear to front as i have heard some rub the underbonnet.)

I run one in a race engine overbore but shortstroked ( 73.5x 76) , as doc mentioned, run a auxillary tank. ( i use the good old moke ones and as advised by a clever mini engine builder/racer, use it as a proper header tank not just an expansion tank) Others are in service in other race engines, so cooling power is excellent. Cheers Darryl

74 degree thermostat, Speedflow 6AN 3/8" Pushlock Aluminum Weld On Barb Alloy Fitting 989-406 ( good ally thickness for welding onto radiator neck in my instance)

these are the ones i used and am happy with for fit and purpose but probably lots and lots of similar....( welding of rad is actually very good as commented by the welder who welded on the barb fitting)
AUSTIN ROVER MINI 1275 GT 1959-1997 50mm ALUMINUM RADIATOR from worley parts on ebay