It is a common snap, you should be able to find a bag of new snaps and the setting tools to
bang the rivet closed for $25.
maybe add a piece of Velcro as a primary closure ,
so You dont have to use the snap all the time to keep the lid closed.
I have the Klick Fix bar bag mount, and their wire bag support frame ..
and adapted other bags to use the support , like a 25 year old Eclipse.
and one of Ortlieb's camera protect bags. it uses a roll and side release buckle-snap
for the closure.
Carradice super C HB bags also use Klick fix hardware. a plate type fitting on the bag.
KF mounts also let me use my ortlieb bag.. an older ultimate 3 RC ,
another roll and snap closure.
so for extra mounts they are more versatile than O's own.