There are a lot of 300S out there, and a very good chance of buying a genuine used FWB diopter here on BBS to replace your broken one.
I have recently bought a spare diopter for my Original Model 75 by asking if anyone had one for sale here on BBS Target/Competition section, and was fortunate enough to find one fairly quickly.
In the worst case that nobody has one they would like to sell to you, it would be possible to get the main diopter body alloy-welded to make a repair. If you can find a model engineer or somebody used to working on a small scale then you should be OK.
I am not sure just how good an adhesive joint would be, but if I did go that route I would use the original 24 hour Araldite epoxy resin as I have found it is still the best epoxy for sheer strength. Once the glued area has been reassembled carefully, it should be held together and left alone until it drys. I use masking tape to hold the parts together and found it to work remarkably well.
Rossendale Target Shooting Club. Every Tuesday and Thursday evening 7 - 10pm.