If it's a D16-rods based D15B, then you'll be fine to about 220hp on stock internals. On a regular D15, maybe 180whp is it. D16A1's (old 'teg motors) have slightly thicker rods, not sure on the DOHC ZC's though.

Anything past that is pushing it. If your gonna build it, just do a basic Vitara Build. You can have all new bearings, most seals/gaskets, Pistons/Rods for less than $700 easily. You still need a few things though.