As jasper co jim pointed out, this website has some tune-up specs, and it correctly shows .015 inches for valve lash.
Incidentally, perhaps a better way to set the valve lash on an "H" (especially with worn rocker arms and valve stem ends that make it difficult to get an accurate reading with feeler gauges) is to follow the instructions shown in all of the various operator"s manuals published by Deere for the Model "H":
"Remove tappet cover on front of cylinder head, and both spark plugs from cylinder. Turn flywheel until left-hand piston is in extreme forward position and both tappets are loose. Adjust both tappets on left-hand side with engine in this position. Turn flywheel until right-hand piston is in extreme forward position and both tappets are loose. Adjust both tappets on right-hand side with engine in this position.
Adjust tappets by tightening adjusting screws until there is no shake in levers, then back off adjusting screws ¼ turn. Be sure adjustment is locked tight. Turning the adjusting screw back ¼ turn gives .015-inch valve clearance."
Regarding your lack of power, you should probably also check your compression and the timing of the mag (should fire 30 - 35 degrees before top dead center).