That bolt holds the rider foot pegs, so has to reach all the way through the frame, with some to spare at both ends. Usually threaded at both ends, with a nut and washer on each, but some may have had a hex head on one end; just makes it harder to get one peg off. If you don't get an answer to the original length, you can measure your frame, and allow for the plates, nuts, and washers. You can use threaded stock, cut to length, but it is not optimum, as the threads will contact the channel in the engine bottom case, and could gouge into it.
I'll look into my old take-off parts, and see if I have one. If I do, I'll measure it and let you know; I'd mail it to you, but that may be expensive.