Like Sunrise says, filling them back up with pox will work. If you don’t have access to M6 tapping tools you can try to dip a screw in PVA, you can make a PVA solution by diluting one of those dishwasher tablet wrappers in a spoonful of water. Make sure the PVA solution is not too slimy as that will go at the cost of detail of the to be molded thread. Dip in the screw and let the PVA dry up, now there’s a water diluting release film on the thread.
You may need to carefully ream the hole with a Ø6mm drill bit to remove some old thread.
Fill the fin hole with pox and press the screw in to the wet resin. After curing, just immerse the fin in hot water to let the PVA dilute once more and turn out the screw.
As an alternative to Epoxy you can use an MMA 2 component adhesive, this is stronger.