If you mean the top screws that hold the engine onto the engine mount....
Take the engine out of the car using the screws on the bottom of the chassis. Dremel a slot across the stripped hex head using a cutoff tool and back it out with a screwdriver.
If it is a screw on the bottom of the chassis you can dremel a slot in the screw, but you need to use a small cutoff wheel so you don't cut into the chassis.
Take your time and use a steady hand