I had mine apart just a couple weeks ago... it was either 12 mm or 14 mm.... I'm leaning towards 14 mm but am far from positive.

My motorcycles use a lot of big internal hex fasteners. What I did years ago was buy 10, 12, and 14 mm hex wrenches and cut a couple 2" lengths out of each. Slide the stub into a standard socket and you have a wrench. Very inexpensive!
I used an abrasive cut off and frequent cooling to cut them without softening the steel.

Short of that, yes, a longish bolt with the proper head size will work. Put a double nut on the end so you can attach a socket and torque wrench.