In terms of dollars per horsepower, installing aftermarket performance parts on the stock motor doesn't stack up to the power you'd get from doing a turbo conversion. You could spend thousands of dollars on high compression pistons, stronger rods, headwork, big valves, camshaft, carb, etc, and still not have the kind of power that you'd easily get from a G54BT.

95 Montero SR
3.8 MIVEC, Advance headers, 2 1/2" exhaust, Magnaflow muffler, OME shocks & rear springs, 2" body lift, 3" tank lift, 4.90s, TRE front locker, factory rear locker, Roger Brown Rock Sliderz, 315/75r16 (35") tires, Sport big brakes