Overheated engine/loss of power

In that case then I would agree that the engine is producing less power.

If you have a fixed pitch prop then speed is proportional to rpm which is proportional to power. So unless you have a variable pitch prop or a dodgy rpm gauge or ASI then I would say that you have a different prop pitch to the one that you were used to. As has been suggested I would chock the aircraft and see if the maximum static rpm is the same as it was before.

Prop pitch can definitely make a difference in many things. Too flat or too course isn't good and prop pitch and length should be set to match a plane and engine that it's mounted on.

What is your wide open throttle rpm at your cruise altitude? This will rule out one issue. Fuel delivery can be a factor.

These two are maybe the most common causes of reduced or increased rpm, fuel usage, engine temps, climb and cruise speeds, ect... Even improper carb setup may affect your outcome.