fist off do you think a stang pg would fit the duo and how about the control plate and mustang bridge ? Control plate shouldnt be much of an issue, but the rest im not so sure.
how cheap could i get a guard for ? Real stang gurads arent that expensive,15-35 depending on brand/colour, but as its not a real stang you may need a custom one, so i would budget for around $40
and how much do you think i could get it repaitnted for ? Depends if you strip original finish yourself or not,and what type of finish you want, i would budget for $200 stripped, $250 if you let them do it.
All in all, its a lot of work, im sure its all doable, and would make for a nice guitar, but a lot easier to sell the duo and buy a stang IMO.