My brothers 29 AA tanker truck overheated w/ cherry red manifold

[Neil Mylar LakewoodCA]

My youngest brother owns a 1929 AA tanker truck. It's been sitting in his back yard for years (see pictures). It would overheat and get so hot that the exhaust manifold turned cherry red. He removed the muffler to see if it made any difference and it did not. He removed and cleaned out the radiator. Made no difference. He removed the timing cover and found that the spring inside the plunger was broken and that the timing gear was probably moving in and out. He wondered if that could have changed the timing making it overheat. The truck has been sitting for years with the timing cover still off and a plastic tarp over the motor.
Have any of you experienced a cherry red exhaust manifold and if so, what was the cause?
I wonder if at one time a squirrel or a mouse had stored some nuts or food in the motor's water jackets?
Any ideas would be much appreciated?
He is thinking of selling the truck, but wants to correct the overheating problem and get it running first.