I have two Vanguard Griptonite stocks that I replaced the factory sling studs with the Uncle Mikes style flush sling swivels. Both the front and rear swivel sites had to be drilled out 1/2" diameter, and I reinforced the inside of the stock with epoxy.
I did the same thing on other plast
In the case of the OP's stock, the broken sling studs can be removed, then reinforce the inside of the stock with epoxy and screw in new sling studs.