What is wrong with the sprayer? Just fix whatever is wrong. It is very simple to fix the plumbing, or the pump. Pumps go out frequently if you buy the Fimco, or other cheap crap. Go to Northern Tool and buy a North Star pump. They have a three-year warranty and they honor the warranty very well. The hose can be picked up in many places. I do some commercial spraying and I have plumbed three, 25-gallon tanks together with separate valves, so that I can use them one at a time. The 75 gallons does not slop as badly, as if it was one big tank.
Mine is for a Kawasaki Mule Pro FX, though, and of course not for the four-wheeler.
If you are bent on getting a new one, just go to a farm store and buy whatever they have. They are all about the same, unless you want to pay higher money for a more customized option someplace.