With the amount of adhesives available today, If you found a Monza style cap that would sit over your existing fuel tank spigotI am pretty sure you could glue on in place.
Many years ago I fitted a Monza cap to a m/cycle tank using cataloy/bondo, I just built up the cataloy around the filler neck and pushed the new filler cap into place, wiping away the excess cataloy as it oozed out. It worked a treat and gave no problems in service. I suspect the lip around the filler neck and the cut threads on the inside of the Monza cap gave the cataloy a good grip to both.
If that task was tried today whatever substance as both adhesive/filler would have to be resistant to the fuel in use... I was a teenager when I did the deed, life was just not that complicated back then..
