That's a fairly unique bolt as it's not only hex head, but I believe a square profile of the head. I needed one like that for a Lionel steamer, and I made it by grinding down the head on a pan head bolt. It wasn't perfect, but unless you looked closely at it, you'd never know.
Here's one that's close at McMaster-Carr. They don't have 13.5mm, but you can knock off the extra 1/2mm pretty easily. You can also probably take some thickness off the head to better match the application.