a CHEAP alarm might be possible, but it might also, trigger a lot for no real reason

Harbor freight sells drive alarms that work off motion, there like 10 bucks, and are pretty LOUD when on high setting
I use them at a few propeties I have, and they do work well for what they are

BUT to use one on your atv, you will have to leave the speaker within about 50 yards in line of sight of atv, you can test to see if you an get it farther, but on a calm day I would think for sure you could hear it at a 150-200 yrds away or more maybe?
you would need to place the motion end of it, maybe inside a fender or wheel well, so it doesn't pick of small things to trigger it, like leaves blowing or birds flying by, or just someone walking past it!
as I doubt you want it going off when your hunting for no real reason spooking game on you!

they ONLY work when motion is in front of them, BEEP a few times then stop till motion comes again!
but it MIGHT help keep folks away if it went off when they moved the atv or trigger it< and again I would think if its NOT real windy, you'd hear it
the real problem is, if folks have BAD intentions and wish to damage your atv, cut wires or??
by the time you covered the 150-250 yards to get to the atv the damage will be done and odds are them long gone, folks that do damage like this don;'t tend to stick around, all the more so if they know the possible owner has a GUN with them or other weapon!
again for 10 bucks, it MIGHT be worth trying if your really set on having something

as for folks jacking up tires to steal ramps, I am sure some will be willing to do so, we were just lucky no none did for us, but did have a LOT of folks ramps stolen when they locked or chained them down, as again, bolt cutters are easier and faster for them to cut grab and go, over just jacking things up
windows on trucks have been smashed in the past and items in trucks stolen as well
LOCK and things only work on honest folks
crooks will do MANY things to get your things if they want and most don't take a lot of work , if your honest and look at things
best youc an do is IMO again, carry full coverage insurance on it and your vehicles!, sad, but can save you a lot of stress and out of pocket loss at times when they do!
I have had 3 vehicles broken into and things stolen over the yrs and one snowmobile and a trailer!
found out the hard way once NOT having one insured and will never again NOT have insurance on things that AIN"T cheap to replace!

AS for parking an atv CLOSE to where your hunting
I know guys that park them under the tree there in/next to it and kill deer
I have also seen deer see a ATV and avoid it, and I have to think they sure can smell them with fuel in them , exhaust fumes and so
a BIG part of this, is, how YOUR deer react to atv and scents of the likes
some places atv's are super common and I think deer adjust to them pretty well
other places, they might never come near you or your atv(as you too will get smells on you from atv riding it into your stands, fumes from exhaust will transfer onto you as you ride it)

its a personal thing, some care more about some not at all
deer ain't rocket scientists , either, , no matter what many want to make them out to be, some smarter than others YES< but many dumb one out there too LOL