Pull the heads the valve seats have come out holding the valves open. Most likely the exhaust valve seats, but could be intake. this is a common problem on the opposed twin briggs. If the seats haven't worn the block too much they can be put back in and peened back into place to hold them. i suspect that one had come out some time ago and was running on one cylinder and then it died after the seat came out of the other cylinder, so the first seat may have hammered the block pretty good and may not be easily repaired.
