Welcome to the board 295WW...
I'm going to guess that your hairline crack goes vertically from the bottom of the muffler hole in the side of the body down underneath the machine. There are a lot of options out there to fix it and make it waterproof.
Options include:
*plastic welding (check in the yellow pages, a lot of body shops weld plastic with all the plastic bumpers and fascias out there on vehicles today)
*ABS slurry (taking shavings of the ABS and making a slurry with acetone and patching up the hole that way... not always as strong as original)
*Rivet steel or plastic (you can rivet a piece of steel or ABS/ HDPE plastic cut to size to the body. Some clear silicone will help in the waterproof-ness, and if you take care, it doesn't look altogether botched-up
There are probably other options out there, too. I have a machine that I'm retrofitting a 1/4" thick HDPE skid plate too which will cover the entire underside of the machine. I'll lose 1/4" of ground clearance, but it should last the lifetime of the machine. I plan on posting the whole effort as an article, but I'm moving slowly with other projects, and I need to go get some waterproof Hey_Dan rivets to get it all done up right. Here's a pic after about 150 KW/ hours with a heat gun and careful application of force with jacks. You can basically just melt it into place. Most of the work is done by gravity sucking the machine toward the center of the planet, and into the soft plastic.
ABS doesn't have the abrasion resistance of HDPE, and is a bit more brittle than HDPE. HDPE tends to be a bit more ductile. I like ABS because it'll shine like the surface of the sun though, whereas HDPE is tough to get past a dull glow, especially with the surface texture.
The best way to get a shine back to your ABS plastic (also known as Royalite 20) is to sand it. Yep, it sounds weird, but sanding the yellow ABS is really a piece of cake even if it gets boring and tiresome after a while. The results pay off. Depending on how bad the UV damage is, you can start with between a 150 and 220 grit sandpaper that's suitable for wet-sanding (it'll say it right on the paper if it is). Use a sanding block for big flat areas (side, top, etc.) and fold the paper over and just use your hands for contoured areas. Keep the paper wet so the water you're sanding with keeps flushing out that what you're sanding off. You'll see results immediately. Do the whole machine with the first grit, then step up to finer and finer grits until you get the desired result. Some people go as high as 600, but I say a solid 220, rubbing compound, and a good wax will look great for a long time. For goodness sake, it's an ATV, not a
Concours d' Elegance candidate.
Keep machines waxed, as this helps stave-off UV damage, and keeps dirt, grease, oil, gas, etc. from staining the body. Plastic, especially ABS is awfully porous. You can close up those "pores" by sanding and waxing.
Show us some pictures of your machine. I bet that if it's not too bad (most machines aren't) you can make a tremendous amount of progress in just an afternoon. Get a six pack of
<insert adult beverage here> , turn on some
<insert classic rock band here> and start sanding away!
Good luck!
~mike