Pardon me, but is that actually the faucet? A picture taken from a little farther back would be helpful also. IF that is the faucet, and IF that is the incoming hot water line running vertical on the left - - could you shut off the water supply, remove the bonnet/hex nut and remove the broken screw from the stem that way? I thought I had seen a lot of older faucet's but that's a new one to me. Good Luck, David