Cool Greg. You may get the fourth bolt pretty easy by putting a 16 inch 1/4 drill through the ring and firewall near the problem one and screwing a brass insert into the new hole in the firewall from the outside. Most good hardware stores carry them. Had some good success with chasing the threads on the existing t nut too. Many times they come loose from the firewall internally and spin though. Pulling outward on the tap using vise grips will usually prevent them from spinning once you engage the tap a 1/2 turn or so very gently. I think that t nut is 4 mm. Not 10/32. Taps are cheap. Another way to go is put a light music wire through from the outside till you can get to it on the inside. Push the t nut out and install the new one via the wire.
A simple request on the general section in the forums will usually get a response from someone with parts like me almost immediately. I personally have given away nos parts and recieved nos parts free or for the cost of shipping someone a mailer. Heck I given away three cowls for the Gp 1359 and gp 1349 myself. One with the fake radial in perfect condition. Also seen them sell more than once on RCU for 30 or less. Monitoring your post doesn't take any time at all.
There are lots of people like me that have a bunch of these common discontinued great planes models out there. There were a LOT made and sold for many years My buddy got one for 300 this spring nib. I haven't ever paid more than 350 for one flyable with some kind of motor I tossed in the scrap pile so I could mount one of my radials on it. Then pitched the cowl in a box till someone needed one. Or the trash when they pulled up.
Don't give up to easy dude. Big world out there, lots of good folks in this hobby. And a few scamming stinkers as with anything else.
Bottom line, just watch the three you have in there with good split ring lock washers next to a flat washer and fly on brother!
Battle scars are character.....