I see this too often on SxS's too. Sit in the creek/river/lake/ocean for x number of days/weeks/months. It's just water, so let's just change the fluids. Right?
WRONG!
That water has mud, dirt, poo in it. It settles to the bottom of the container, the container being the engine, transmission, everything it touches, it settles into. You can change the fluids 2,000 times and never get it all out. The ONLY foolproof way to get it all out is to disassemble the "container" and manually clean it all out. If you don't want to do that, chances are good that at some point that dirt will dislodge from where it is currently and wreak havoc on stuff that is intolerant to dirt.
On the ATV/UTV stuff (and tractors too) the container (engine, trans, differentials, etc) has many nooks and crannies that the dirt can settle into. Over time it breaks loose and circulates throughout the engine. Sure I can change the oil 4 times on a Wolverine 850, and it stays clean for a while but sometime in the future, might be next week, maybe next year, it WILL start smoking, knocking, rattling, or just kick a rod out due to the dirt that broke loose and became sandpaper inside the engine (or transmission or front diff) They (engines and transmissions) have filters but they don't do much good when the dirt breaks loose while running and that dirt is slung around inside by the rapidly spinning crankshaft, which turns the oil into more of a froth and slings it up into the cylinder walls, camshaft, tappets, --and some of it gets into the lube circuit and through the head(s) and crankshaft and balance shaft(s) too, which usually destroys them. That's why I advise owners of SxS and ATV to avoid deep water. Whether they listen or not, is up to them.
That said, it's entirely possible that it's already done that. I don't mean to be the bearer of bad news but I think you've got yourself quite the project there. Now that it's been sitting that long with dirt/mud all over it, and inside it, you're gonna have a time of it getting it all off and out.
If it were me (and admittedly it is not), I'd have never bought it in the first place, and for some dumb reason I did, I'd have it cleaned off and sold in no time. I never ever ever ever ever EVER buy flood vehicles, and I won't take them if someone paid me to take them either. They are scrap to me. All of them. And I buy about 150 or so ATV/UTV a year, but I will not buy flooded/sunk ones. Just too much work for very little if any money to be made. Usually a loss, TBH.
