Harley Davidson Street 750 accessories price list for India

I don't think changing the caliper will get you much. Increasing the diameter of the rotor is more effective. Find a larger rotor that matches the Harley bolt hole pattern and then have someone make an adapter bracket for your existing caliper. A tech friend does this with older BMWs, using the old calipers with late model BMW 305 mm rotors and braided stainless steel brake hoses and the resulting brake performance is great by any standard. I did something similar on one of my old BMWs, but I used some old Brembo P432D calipers I had lying around from another project (same caliper Ducati used on the 851 Superbike and pretty close to what they hang on many modern bikes). A local machinest made brackets to mount these on the stock forks at the correct height and offset for the rotors, stock BMW rotors from a K1100 mounted on an old K1100LT (light truck) front wheel.

If you replace the stock caliper with a four piston Brembo caliper, you will have to replace the master cylinder with one that is 1 mm or 2 mm larger diamter, maybe more. Going from two to four pistons means you have to move more brake fluid to squeeze the pads against the rotor. If you don't change the master cylinder to match the caliper(s) lever travel will be excessive and you might run out of lever travel before you can get full pressure at the caliper.