S1 is quite thin (approx 15mm, same as R33 GTST IIRC)
An aftermarket koyo rad (standard construction, plastic top and bottom tanks) is thicker at about 25mm, but has fewer flutes than the stock rad, as the flutes are wider.
No idea which would flow better, could be a close call (fewer large flutes v's more smaller flutes) but surely additional thickness will improve cooling?
Y31 Cima rad is identical in width/height as the C34/R33 rad, but is double row, so a good upgrade for cheap if you can get one!