You could do a regular heart prop and make the beating effect using a flashing light/sound, or have it mounted on a flexible surface and conceal a small motor to move the heart to simulate it beating (I'd try doing a piece of cloth, tightly stretched over a open box and maybe hot glued so it looks like a solid surface. Attach the heart in the center to the cloth (few dots of hot glue? Needle and thread and put a few stitches in to hold steady?). After securing the heart to the middle of the cloth, put a rotating motor with a level/paddle that will nudge the heart on a slow RPM in time with the light or sound effect.
Or you could get this type of heart that has a color changing effect and just have a small sound loop of the beating hidden nearby (mp3 with a mini speaker)