Soundguy is correct. The job is easier if a person removes the engine cowling, ("tin") so you can remove the radiator so you alleviate the possibility of damaging the radiator which is an expensive item to fix or replace....while you replace the water pump. That way the water pump is out there in the open and much easier to work on. BTW, get some help lifting the engine cowling off the machine so you don't hurt yourself or the tin, (unless you're a burly enough feller and don't have any help). It's a process, takes longer, but, in my humble opinion, worth it.
Assuming the 900 is a narrow front end like a 961, and I think it is, then...the radiator has two bolts that goes through the bottom of the pedestal. One nut for one bolt is above one tire, other nut for other bolt is above the other tire. Soaking the nuts in some penetrating oil for a while may save you some grouchiness. Believe me, you will be grouchy if you spin a bolt head in the bottom of the radiator. Remove the nuts, being careful to not apply too much force to keep the bolt from turning which is held in the bottom of the radiator. In other words, the bottom of the radiator has a "slot" for the bolt head and is supposed hold or keep the head of the bolt from turning and therefore the nuts should be snug/tight when reinstalling. But be careful here. That's a good time to keep the burliness in check, when removing and installing the radiator mount bolt nuts. Radiator metal is thin and easy to damage so use caution here, believe me I know cause I done it. It may be a good time to replace the radiator mount pads. You can decide that.
It goes without saying that a person will need to remove the radiator hoses.
As I said, once the tin and radiator is out of the way then you can see just how easy it should be to replace the water pump.
If I have forgotten an important fact then I hope a more experienced person will jump in and please correct me or add to what I've written.
Post back and let us know what you come up with Redman.
Hope this helps and Good luck.
LW, Tractor Rookie.