I'd get a thread file first and try cleaning that thread up. The thread file will tell you what the pitch is, Hopefully the threads will come right and a new genuine nut will do. if you need to find a nut,you will have to make an educated guess what size it is by measuring the diameter. It will be a metric diameter and although most metric threads are whole mm steps you can get special threads in half mill, so measure carefully. You won't get a left hand thread in any common tap and die set, but they can be bought quite easily individually if you really need one. If the tread file won't clean up the threads though then neither will a die nut. And yeah, zap guns are hard on threads always and shouldn't be used for routine undoing stuff. They spin too fast and promote galling even in good threads.

I had crank threads buggered up on a Yamaha DT 400 I was building, I measured with mm ruler, of course your tap and die set never has the one you need, but was handy measuring the thread pitch. I then ordered the one I needed to straighten the threads up, it worked, of course machine shop would be better but I didn't want to haul the bike there to get it done. If you have calipers you could definitely measure it with that, I did that also, but mine were not really expensive ones so I double checked with mm ruler. Will your nut start back on there at all. You might get away with just buying a new nut, get two, get it started and let it straighten out the threads.

Yeah, we can inspect the chain for wear, and the slippers, and be pretty certain whether they are ok or not. The tensioner is the tricky bit. And that backfiring throwing the chain or timing belt is well known. Some motors they warn you against rotating backwards because the belt can jump.

I was also thinking faulty tensioner, plus the rubber piece wears and causes it to not be as effective.

Check the ratcheting mechanism on the tensioner really carefully.. If a motor backfires or bounces backwards as it's coming to a stop, the tension comes on the wrong side of the chain and can subject the tensioner to pressures it wasn't designed to take.. A defective ratchet can fail under those conditions..

Inspect your gears really closely for damage, I generally like to put on a new cam gear when I replace the chain. Im a little concerned as to why it came off, they can be noisy for a while and not come off, just inspect everything down there really closely when you get it apart