I have an old MTD model 990 Farm king garden tractor from 1977. I've been having overheating proplems for about a month now. The engine is the original 16hp horizontal briggs model #326432. All of the following has been done to try and correct the problem:

Oil change

Air filter replacement

Carb adjustment

Replaced head gasket(twice)

cleaned all debris from the engine

The exhaust has been getting so hot I've had to stop cutting grass every half hour or so to let the engine cool down. What are some of the other obvious causes of overheating?