Yeah, like the other guys said, just buy purified or distilled water. Consider long-life conventional (green) antifreeze. Thats what I use.
Once you have your system drained, it might be a good idea to inspect the cooling system CAREFULLY. As in the lower radiator hose and any leaks. The engines vibrate a lot on these rigs, often the bottom sides of the radiator mount (where it's soldered to the radiator) will pull away after several years. Check carefully that yours is secure. You can easily re-solder the metal mount to the copper radiator with a torch if need be. If it is loose and left alone, it can lead to radiator cracks or worse. If you have ANY doubt about the lower radiator hose, replace. What can happen is, if it sits for some time a few drops at a time on grass or gravel, go unnoticed and can drain the cooling system between uses and you'd hate to mistakenly overheat your engine. Of course always check coolant before you run your skid. If you park on concrete than you'd probably see a leak.
Also the fan shroud mounting tabs often crack and may not be holding the shroud as secure as it could be. Give them a look-see. There have been more than a few Case owners who've had floppy shrouds eventually get into the fan and before they could shut the machine down it can ruin a blade, radiator and other expensive parts. The tabs on my shroud were problematic before I fixed them. There was a guy here years ago who had a shroud get sucked in ...and soon after I checked mine ...glad I did.
So .....just give the rest of the system a look over before adding the new coolant so everything else is good to go for a while, and you'll have a happy machine!!
Good luck