actually, with parts available on the market, I'd bet on a V6; No replacement for displacement, and while I'm not sure about the MZ, I know the 3VZ can be stroked to (possibly) ~3.8 litres
put a turbo setup equal to the setup you'd mount on the 5S/3S, and you'll have proportionately more power
and the V6 stock performance is equal if not better than the stock MR2 Turbo, due to the presence of torque down low in the power band (again, 3VZ. I'm assuming that the 1MZ has a vaguely similar powerband...)
on a tangent, the V6-MR2 is actually my backup car idea, should anything happen to my camry (that doesn't destroy the engine)