With the caliper still on the car use pressure generated by the brake mater cylinder to force out the caliper piston. To do this remove the brake caliper from its mount and get an assistant to pump the brake pedal gentle and firmly to generate pressure, keep your hand clear of the caliper piston as the force generated is huge. Also use precaution as brake fluid is damaging to painted surfaces, and harmful to your eyes. If the caliper piston can not be with drawn from the caliper using master cylinder pressure you will have to remove the caliper. I have made a fitting that allows me to use a grease gun that fits to where the brake hydraulic hose would enter the caliper and use the pressure generated by the grease gun to force the piston out, again making sure my fingers are well clear of the piston. There are people you can find on line that professionally rebuild brake calipers and it may be prudent to contact them or your local parts supplier to see how much a set of re manufactured calipers would be or if you could have your own calipers rebuilt. Do both front calipers not just one so that you have similar brake efficiency and braking characteristics from both front calipers. You may want to inspect or replace your front brake pads and brake hydraulic flexible hoses at this time and flush the brake fluid out from all four wheels.