puchfinnland

I work on boats and had the same thing on a rivnut on a mast.

the bottle bolts are most likely M5.

get a m5 bolt, and an m5 nut,

thread the bolt and nut in the loose rivnut,

hold the bolt, and then tighten the nut onto the rivnut, as you tighten it you compress the rivnut and it should do the trick,

now if the original bolt is spinning around in the rivnut and it cant come out..

this requires mechanical methods,
to save the paint- grind off the head with a small dremel tool, and be sure to tape up that frame well to protect,

when you have the head gone you can continue carefully untill the entire thing drops in th eframe.

I would have the fork out and the bike tilted forward so the debris fall out the front.

the bike shop can put you in a new one- a bit of polyurethean calking in the hole BEFORE he puts it in will help keep the rust away .

hope this helps