I have the same bike but checked on-line at
www.mrcycles.com. Apparently it is a 10mm bole 28mm long. These are probably available with coarse or fine threads, like inch bolts but as long as you get a hex nut in the same pitch, the assembly should work and the bolt will go thru the hole in the frame and the muffler support bracket.
You will find it useful to be able to look up parts for your bike. The Honda fiches (a kind of high res photograph of a printed parts page) give some details of the parts often like the sizes of bolts and o-rings. What you will see at
www.mrcycles.com and other places are reproductions of the Honda fiches. Maybe
www.mrcycles.com is not the best one to use, others may chime in with their favorites?