Pull the thermostat and put it in hot water. If your thermostat is like mine(1977 75hp Evinrude) the t-stat should start to open at 145 degrees.
If this is your motor...it's #12
http://www.marineengine.com/parts/j...rer=Evinrude§ion=Cylinder+&+Crankcase
You may end up needing a new gasket(s) after you pull off the housing.
You talking about a new head gasket? I have to pull off the cylinder head?
No. The t-stat housing gasket. Mine was in bad shape and came off in pieces.
Doesn't hurt to check you thermostat regardless of whether it's the cause of problems. If it's old, it's not a bad idea to replace it. They're only $20 bucks.