You only have a few choices. By mearly adding a current limiting device between the transformer and the 50kAIC breaker doesn't help. They must be series rated together, which they generally are not for older equipment.
You can add a reactor downstream of the transformer, and it will limit fault current, but be careful of voltage drop problems.
Validate the fault current that you are being told, at least with an infinite primary calculation, and don't forget to add contribution from downstream motors.
Wire can reduce current significantly, so run lots of wire as mentioned by JBD.