That is an ESKA; one of their few water-cooled varieties. It is either rated at 9.9 or 15 HP depending on the carburetor and prhaps some other variables.
One word of caution: :NEVER rope the motor over with the spark plug wires de-tached. The ignition module is VERY sensitive to that and will fail soon afterward. DON'T DO IT!. Always keep the spark plug wires grounded to the plug or engine block.
