Best type of jb weld kind of product for gas tanks

I think they make one for gas tanks. But sounds like yours isn’t leaking, just some acre hole issues? But you need one that binds to the polyethylene or whatever plastic they use. 

This is a tricky one. Is the Fetzer valve damaged?

Someone got excited and way overtightened things. Bummer.

For the petcock holes that the eJot screws go into, I don't really fix them. I take a clipped end from a zip tie, lay it in the hole cut flush so it doesnt lift the petcock up, then run the screw(s) in.

For the brass seat retaining insert you are more screwed here. Can maybe wedge/pack some metallic epoxy in there to keep it from spinning, but won't take much to mess it up again. J-B Weld SteelStik or similar here.

Best way is to plastic weld it (filler) but for fuel tanks if it isn't the exact same stuff it usually won't work. When I worked at a dealership we had a few damaged fuel tanks stashed on shelves we could take some shavings from to feed the plastic welder.

Plastic welder?

I could’ve used that when I was trying to fit a ‘94 Cr250 tank on my ‘94 Cr500. The bottom of the Cr250 tank was touching the 500 jug. All it needed was a little taken off the bottom and re-welded (plastic).

I use jb weld on the brass inserts for the shrouds, works fine.   Getting everything flush for the petcock is a bit harder.

could be the bypass line? 

I was thinking that, also. It’s really nothing that some 3-in1 oil, heavy gauze, and some ball bearings can’t handle. Still more realizable than that JB weld stuff IMO.

So did I. As long as have the bolt out it should hold. You are not gluing the insert in place but locking it in with JB Weld. Add the JB while it's a little bit runny so it fills any voids between the tank and insert.  

Are either of you gentlemen familiar with the Turbo Encabulator or its predecessor the Retro Encabulator? 

Yes! Big fan.

Marine-Tex & clean up the hole for the insert with a Dremel use a bolt with anti-seize to guide. I have done this many times over the years

I dont know the exact setup you are working with but I have used these to repair shroud bolts in a tank before, they use an adapter for a soldering iron to heat them up and then you just push them in. 

My dads a retired plastics engineer and I discussed with him multiple times on stuff like this, the plastic they use for tanks is virtually impossible to get a good bond on but there is a glue called Methyl Methacrylate that will do the best job its expenisve and a hassle but its the best. 

A new tank is about $150 so depending on how far you are traveling and spending money to race chancing a repair may not be worth it but thats for you to decide. 

I'd think a seat bolt (seems like a strange engineering choice?) threaded into a plastic tank is a different problem from a shroud bolt (which on their own pose real challenges) because of the higher torque on the bolt needed to keep the seat secure and the forces and weight that are transferring from the seat to its forward attachment point, the bolt and fuel tank.  My experience, only with shroud bolts,  is that you have a low chance of a permanent fix and a high probability of losing the seat mid-moto on the bike at some point.  Not a great option, but if it were me, I'd get a new tank.