I have run into this problem with older motorcycles.

Harleys from the fifties and old Indians and Hendersons all used a bolt that is thread at 1/4-24. For Indians they are used to hold the skirts together so it is no big deal to replace them, but on Harleys they are used throughout the casings almost everywhere from holding on the generator to holding on the primary cover. For a long time they were hard to find but thanks to the upsurge in popularity among old guys trying to recapture their youth, they have become available again although they are expensive.

What we used to do was go to small machine shops, little one-off job shops and have bolts made but they have all seem to have disappeared with the advent of CNC shops.