Small Engines (Lawn Mowers, etc.): Muffler Glows Red, oil viscosity, lean air

Question
I have a 20hp Kohler Courage engine that had seized up and was disassembled. The lower bearing was scored.  I cleaned up the bearing and reassembled.  It runs but the muffler glows red, the power seems weak (it never had good power) and it sounds different now.  Note the engine has a total of about 27 hours on it.  Note also that it starts a little hard, but it always has, even before it seized.  Although it was under warranty at the time it seized, the cooling fins were blocked by a mouse nest so Kohler would not cover it. There was no damage at all to the piston. or cylinder.  Any ideas what could cause the lack of power and glowing muffler?

Answer
Jim
Sounds like the timing is off Possible causes:

Magneto is not timed properly.
Carburetor set too lean.
Air intake or cooling fins are clogged.
Shroud or blower housing missing.
Excessive load.
Insufficient or excessive oil.
Improper oil viscosity (4 stroke) or mixture (2 stroke)
Valve clearance is too small.
Excessive carbon buildup in combustion chamber.