Re: 15 hp force
Torque specs for what? If it is the head and it has 1/4-20 bolts, torque is 70 inch pounds. If it has 5/16-18 bolts I SUSPECT that the torque would be the same as larger engines--225 inch pounds.
As with any fastener, if in doubt, look up and use standard torque values as listed in supply catalogs.