Here is what happened; the cowling now has plenty of cooling for the engine.  I bought a 16 by 10 prop.  The engine now runs good at high RPMs, but it is now stalling at low RPMs.  We had it running good on the table at my club, someone was carying it to the runway and it stalled just before he set it down.   I took the engine apart; the piston is silver on the side but brown on top.  Have I done permanent overheating damage?  If the lower part of the engine was buried in the cowling, shouldn't the sides or lower part be overheated?  And not the cylinder head?
I have so much time invested in this plane I'm thinking of buying a new evolution 120 and put this engine in somthing simpler and cheaper, maybe a used plane.  Do aluminum pistons normally turn brown on top?