This is the idea that comes to mind
I'll try to explain, excuse the English. Hope that the diagram helps to understand better.
The idea is to drill a small hole where was the blade that broke. It should be in line with the remaining blade.
Fix a long screw in this hole and screw in it some nuts as a counterweight. Adding more nuts and/or moving the nuts in or out of the center would balance the propeller.
Once the final adjustment is achieved, the screw would be threaded and finally glued to the propeller and the nuts would be fixed by pressing them against each other. Or it would be fixed with glue in case it is a single nut.
I have never tried it, but it should work theoretically. And the balancing process does not seem to be very difficult.