Well it could be many things but here are a few to ideas to start.
Is the rad clean under the oil cooler?
The belt that drives the fan is on a spring loaded tensioner, somtimes the tensioner seizes up and the belt slowly gets loose, which prevents the fan from turning full rpm, thus reducing air flow through the rad.
Could be a thermostat, could be a faulty temp guage sender or a wire with bad insulation grounding to frame.
Can you get your hands on a hand held temp gun to confirm the engine is in fact hot?
Is the engine using any coolant? If the head was cracked your antifreeze would likely be getting lower.
If you let the machine sit and idle does it cool down or keep getting hotter?
Ken