I wouldnt wreck a scale wheels locomotive. Theres so few of them, I'm sure it wouldnt be much trouble to list it for sale and get the one you want. (or just return it)
Youd need pilots, couplers and all the hand rails to convert it. Maybe theres someone out there to trade, but for as many SD70s as MTH has run, its shouldnt be too hard to find a 3 rail one.