I am going to make some assumptions here.
First, you have a Saf-T-Liner RE with the engine in back.
Second, if you do have a rear engine bus the radiator is mounted 90* to the engine in the side of the bus.
If those assumptions are correct the fan is operated in one of three ways.
- Fan belts turning a jack shaft that turns the fan
- Fan belts directly turning a fan
- Fan is hydrostatically driven by oil pressure from the power steering pump or automatic transmission.
In all three cases the fan is thermostatically controlled to only turn on when it reaches a set temperature, the exact temp I don't recall.
Some of those are pretty basic and easy to find Horton units.
The hydostatically driven fans were Thomas OEM units and not quite as easy to find.
On most rear engine buses the health of the water pump will have little to do with the operation of the radiator fan.
You can override the fan's thermostat by welding together the inner and outer rings of the fan drive. The down side is the fan will be operating 100% of the time. And it is critical to make sure the welds are balanced or the bearings in the fan shaft will be ruined fairly quickly.
I have seen quick fixes that have worked to get someone home. Two or more automotive electric fans were installed to move air throught the radiator. Depending upon where you will be driving and how fast and hard you drive will determine if the quick fix can become a permanent fix or not.
Good luck and I hope you keep your cool.