its most likely the oil temp switch if, you manually checked to make sure fan worked
yes there are free service manuals, Others here will help with that, I'm old school and buy hard copy OEM service manuals for my things?

I have a 1999 400 foreman(your 's is almost the same just a 450 motor)
and I have gone thru 3 different oil temps sensors since 1999 on mine,m and only 1 Fan Control unit

many folks just simply ADD a toggle witch to turn fan on and turn on as they feel needed
I prefer things to work as they were designed
so that is also a option over buying any OEM parts here>

IF you do decide to replace the oil temp switch, I suggest you time it with an oil change as once you unscrew it out of back of motor, it will act as a oil drain plug
thus you will loose a bunch of oil even if your FAST,
but your call
its NOT a hard thing to replace
I tend to stand mine on its rear and work standing up, BAD back here so, try and make life easier if I can!
wires run down right side of battery box like on the removable side panel , so access to all things is rather easy

 

Mrbb, thank you - yes, since it is my mother in laws, she would prefer it to work like designed over installing a switch!

 

here is a link to a shop manual https://drive.google.com/drive/folders/1eqVb5JW5em4PBbDrgHsRbhFKH5rFr75l?sort=13&direction=a