The only stuff I've found that will hold up to the gas fumes is a Permatex product called MotoSeal 1. I initially used it to secure the gas cap gasket on my wife's Vespa when I restored that. I had tried all manner of other adhesives, none of which worked for more than a few days at best. I found the MotoSeal stuff on Permatex's website, and eventually found it at a small local auto parts store. It's been holding that gasket for 3 years now, and I also used it as a supplemental gasket on the carb for the Vespa.
My magnet is loose from the cap of my 69 as well, as well as having lost some of its magnetism (sort of like its owner!
). When I get to that point in the re-assembly I plan to stick it back on with the MotoSeal and I trust it will hold it in place.
As a side thought, there may be an epoxy product that will work with the fuel exposure. Model airplane guys might have something like that, and it would have better adhesive qualities than the MotoSeal which is more of a silicone-type stuff.
Steve