Well. Any front caliper from RRC and Disco 1 and Defenders all "fit" as long as they are for solid or vented rotors...
But... If you are buying new, you are best to get the latest version Defender ones. They use larger pistons than Discos and possibly what is on your 90 (it depends). If you are getting rotors at the same time, then you might as well switch to vented.
It more comes back to what you are planning to do. Lots of things crossover, but each item needs to be talked about to determine suitability. You also need to be careful on which brands you purchase. These are Meccano sets.