I had the same dilemma a few years ago. The guitar in question was only ever really used for recording, but it did a great job.
First, rip out all wiring including pots and selector switches. Then, wire each pickup first to an on/off switch, and then to a dual concentric pot. I got mine in Liverpool for £5 each.
http://www.stewmac.com/shop/Electronics,_p...ntrol_Pots.html
I found this a really good way of getting that 'strat sound' (which let's face it, you need a strat for) and getting the SG controllability. You could also add a phase switch to the middle pickup, but it's up to you. I never did because it was a studio guitar and I could cancel hum or get different sounds through effects or the desk.
Hope this has been of some help.