1989 fourtrax 350

When I got this quad it had milky oil in the rear differential.
The rear brake was rusted out and full of water.

I think the water got in through the brake and through the axle housing into the differential?

While replacing the brake I started thinking about the differential. The pinion spins fine the bearings seem Ok.

Would a little water in the diff really hurt it that much?
I want to get this thing on the road and I dont know if i have the tools/ experience to rebuild this diff alone. (the manual makes it look pretty scary)

Should I rebuild the diff to be safe? Or Re assemble as is?
Just looking for some personal opinions.

Thanks.