Usually when an engine loses power like that, it is because the engine leaned
out in flight for some reason. Did you balance the prop and spinner? An out
of balance engine can vibrate the low end needle and actually change the
mixture setting.

Check over the line to the tank from the muffler, and the feed line to the carb
as well for a leak.

If the engine had gone rich and lost power....you would have seen a lot of smoke
and heard the change as well. You might want to check the valve clearances too.
You may have lost clearance on a valve, and lost compression and power as a
result.

I have one of those engines....it likes the OS "F" plug, and fuel with castor in it.

FBD.