First, you can't convert it to the newer style detachable magazine - the machine cuts on the bottom of the action are totally different. (and before some wise-butt chimes in saying "sure you can with a few modification," let me clarify that it's not a simple parts swap).

As for a replacement stock for what you currently have, good luck! Stocks for the old side-button mag release guns are few and far between. Easiest and cheapest solution would be to convert to the blind magazine. Only other option would be the CDI bottom metal (pricey), and then you have to also have a stock custom inlet to accept it (even more money).

Yeah, I'd be going the blind magazine box/Boyds replacement stock route and be back at the range for under $150.