FSA MegaExo crank tensioner bolt, can I use any 12mm bolt?
I'm inferring you have one of the FSA 19mm cranks with a Shimano-style pinch bolt and slot arrangement to secure the left crank to the spindle.

Any 12mm bolt that can make contact with the left crank can do the job of setting the BB preload. Under normal circumstances that bolt does nothing while riding. However, it does have the secondary function of keeping your crank from falling off for a while if the pinch bolts come loose. Maybe a random bolt and some washers would do the same, maybe not.

The 19mm BBs are not the greatest. I do feel like they get a slightly unfair bad rap because usually riders who burn through them also never adjust them. It's somewhat a matter of opinion whether it's worth it to keep 19mm FSA cranks going. New cranks tend to give you a relatively good deal on fresh chainrings, so that's part of it, but making good use of them needs a new chain and cassette.