The small gauges are 2", a very common size. Practically anything that looks like the same size, is. I ran with a cheap aftermarket mechanical temperature gauge for many years, which fit both the dash and the Tstat housing perfectly. Had to run a new connection for the light, though, as it had it's own bulb instead of the window and external bulb that the original TR2/3 gauges used.
I think this is the one, but not positive offhand
https://shop.advanceautoparts.com/p/sunpro-mechanical-water-temperature-gauge-cp7975/9030093-P
The fuel gauge is a bit trickier, as it has to match the sender. As far as I know, the TR2/3 sender is unique to the TR2/3. But I believe you could use a TR4-6 fuel gauge, by using the sender from that car, plus a voltage stabilizer.
TR4/4A tach & speedo look a little different, but should fit and work. The speedo calibration might be off just a bit but should be close enough (as long as both your car and the donor had the standard 3.7 final drive ratio). Early TR6 speedo would also work (with the same caveat), but is calibrated for the larger tires. But I think the TR6 tach used a different drive ratio, as did the later TR6 speedo. You'll probably also need a different bulb holder for the high beam indicator. Spitfire & Spridget used smaller gauges I believe. I kind of think the MGB did too, but I could be wrong about that.