Was any work done on the fan connection to the relay controlled by the temperature sensor? If you can make the fan run it sounds like the the fan may be hooked up to the wrong contact on the relay that controls the fan or the sensor is gone bad. If the fan was connected to the Normally Closed or NC Contact by mistake, the fan will stop when the sensor activates the relay. The fan should be hooked to the NO or Normally Open contact then the fan will run when the sensor activates the relay
Agreed it's difficult to troubleshoot an issue without details. Having said that I've kind of gotten use to troubleshooting problems with very little information throughout my 40 year career as a Ship's Engineer
I'm pretty sure the original fan was the belt driven fan with the centrifugal clutch