Dipping the toes in its kinda cold!

You should be ok to replace only the chain. Unless the gears have obvious signs of failure (cracks, deep nicks or scuff marks) they should be fine. Stock gearing on that sled was 23 top and 40 bottom with a 70 link chain, so they have definitely been changed. Stock track was a 136" long x 1.5" high lugs and they used 4, 9 tooth drivers. Since you have 2, 8 tooth drivers I would assume someone has installed a 2" lug track and had to drop driver size to compensate. I did this myself on my 1998 unit, put on a 151" long by 2" lug track and 8 tooth drivers. If you raise the rear of your sled, tie a rope to one lug and rotate the track one full revolution, mark the rope then pull it off and lay it on the floor, it's easy to tell how long your track is. There were/are companies selling suspension relocation kits for these as well. They moved the entire suspension back to get a better angle of attack on the snow. I've done this too and it did help reduce trenching. If this has been done you will see the old bolt holes about 3" in front of the ones used now. Another way of using a longer track was aluminum rail extensions at the rear of the suspension, pretty easy to spot these.

On top of the triple pipes, some other popular engine mods were reed spacers, a peeled head gasket and gutting the airbox. Reed spacers were aluminum spacers added between the reed blocks and the crankcase about 3/16" thick. I've done that but didn't notice much difference so removed them. The head gasket mod involved pulling one or two layers out of the 3 layer head gasket to increase compression. I haven't done this as I didn't want to mess with the engine's reliability. Others have with no ill effects. I have tried the gutted and drilled airbox with and without pipes. The only difference I found was an annoyingly loud ride, no performance gain that I could tell. I've switched back to a stock airbox. I ran triple pipes for a few years but have went back to a stock pipe with trail can. Pipes really wake that motor up but are pretty inconsistent when temperature changes 10-12 degrees C. Given the time involved changing jets, I prefer to just ride.

I saw your other post regarding the roller secondary, I can't speak to that as I've only run the stock Yamaha clutches with a bunch of different set ups. There is a lot to gained on these sleds from clutching. If you can find it, Olav Aaen's clutching book is a great resource for learning about CVT's.

I have the 1998 MM700 that has over 10,000 kms on it and haven't touched the engine. I did all the bearings you're doing a few years ago. Other than that it's been a great sled. I bought a 2001 MM700 used a few years ago and it has 7800 kms on it. The machine is stock and going strong. Survived 2 teenage boys so far, they've both moved on to newer 800 RMK's that haven fared so well.

The major drawbacks to them as mountain sleds you'll find out after a few trips. They are heavy and they don't handle nearly as well as newer machines. If you want to throw them around, you have to hang your butt way out there, and sometimes that's not even enough.