You could try using a good epoxy to glue the crack closed. You might have to pull it open to get the epoxy to flow in. Use surgical tubing or pieces of inner tube to "clamp" the crack closed. Then carefully drill a hole from the inside rear of the stock angled into the wrist and epoxy a length of threaded rod in.

I have one of these guns with the upgraded Wolff spring and haven't had any problems. I hope you can get it running.