Rheostat for swamp cooler?

A customer has a very old swamp cooler that has a 1 speed fan motor. The switch only has "on"/ "off". The fan blows too hard and he wants a lower speed. If I put in a 2 speed motor, I must need a different kind of switch to operate it, but I'm not sure what kind of switch to get.

Is it possible to just wire in a rheostat so he can have a variable speed fan, and, if so, where would I put it? I don't want the pump to have variable speed. Just the fan.

No!!!!!!!!!!

That motor, probably a simple, so-called universal , motor, will burn up if you try to vary the speed.

You would need to replace it with a multi-speed motor, or a more expensive true variable speed motor with speed controller.