Don't remember if it's a captive nut tack welded on the back of the flange? Or just a normal nut?
The bolt you are looking for is a socket head cap screw. It will be metric, looking at the photo most likely an M6 x 1, or a M5 x 0.8. You need to either take it into an industrial supplies place / bolt supplier and get them to measure it or get yourself a set of vernier calipers and measure the bolt diameter, length and thread pitch (to work out if it's metric coarse or metric fine, it will most likely be coarse).
Just eyeballing it from the photo, I'd say you'll need either a high tensile socket head cap screw, M5 x 0.8 and 20mm long, or the same but in M6 x 1.0.
Wouldn't hurt to smear a bit of anti seize on it when re assembling either, and make sure you have well fitting (good quality) metric hex keys