There is no flux core wire for MIG welding aluminum. MIG welding aluminum is with shielding gas only. TIG welding or brazing with flux coated rods could build up the parts, but an expensive waste of time and effort. If you've never welded aluminum before I can pretty much guarantee instead of having built up parts you can try to sand smooth and balanced, you will end up with melted mess instead of clutch parts . Aluminum doesn't grind well. It plugs up grind stones and disks. Aluminum melts before it changes colour. Apply too much heat to an area too long and it goes from solid to melted with no warning. Welding aluminum is a special skill not like steel welding which is forgiving of mistakes .. There is no forgiveness in aluminum welding . Work on getting the motor to run first . If it is ok then look for a replacement clutch assembly.. Trying to fix that one will be an expensive lesson in frustration.

The belt is bad period. Look at third pic down about a 1/4th way from top and you will see gray area on the right side of the belt this I think is a narrower width than the rest of the belt. The width for the belt should be the same all the way around the belt if it isn't it will cause a large vibration. This indicates the engines was running at some point with the driven side not turning and wearing the belt down at that point. The picture of the wiring is about usual for an older bike or one that has been setting up a while. For the wiring I would first pressure wash the bike including the wiring but disconnect battery first. Then inspect the wiring for chewed or broke wires. Unplug each connector and insure they are clean and have the electrical lubrication still in them. If they do no need of anything else. If they are corroded or dirty then use electricaal cleaner to clean and get a tube of electrical lube to use in the connector. Then if you have problems with electrical get the service manual for the bike they will have electrical prints in them.

That could be it. You might want to think about putting a new set of clutch plates. Here's a link to one on Ebay if you're interested. It sounds like the clutch plate for 2nd gear might be slipping. You could also try adjusting the clutch a little bit and see if that helps before you buy anything. I have the same bike. https://www.ebay.com/itm/403335178607?mkcid=16&mkevt=1&mkrid=711-127632-2357-0&ssspo=Mx1SeBALTpe&sssrc=2349624&ssuid=GatOuRqcQdy&var=&widget_ver=artemis&media=COPY

It's a cheap China ATV and as far as information and manual that is most likely not happening. I would suggest finding someone local that has experience working on them and have experience in resourcing parts if needed.

You can check here for some various manuals Alex, seems you are a lot like me in regards to brands/years lol. https://drive.google.com/drive/folders/1GcqS65GYd3yFmEAGsI7LP-eZp2jMRVJU