If the pin is broken clean off, but you still have it, I was able to re-attach mine as described herein:
http://www.xs400.com/forum/showthread.php?t=11214
I owned a Honda 450 Nighthawk for 35 days this summer and, before I sold it to my friend, I made a new "nubbin" for one of the covers. A PO had lost it.
Roughen up the surface of the cover where it broke and also sand the flat top of a "trumpet-style" steel woodscrew with course sandpaper. Then apply a blob of GOOP on the cover area and press the screw head into it. I licked my fingers and shaped the excess around and over the screw head.
Position it perfectly vertical so the screw won't lean as the GOOP dries and leave it for 24 hrs in a warm place (or clamp it if you happen to have one that can fit).
Next day, apply GOOP to the threads of the screw and this time hang it upside down so that when the GOOP sags while drying, it will form a kind of blob on the tip of the screw. Let it dry for 24 hrs.
I had to do this several times over several days in order to build up a large enough blob of GOOP in the right place (don't use too long a wood screw, but do use a thick one). The last day, it was actually a little too big to go in through the rubber mounting hole and I had to trim it back slightly with real sharp scissors until it fit very snugly.
I know, it sounds stupid and awfully fiddly, but it worked perfect and should last a long time since it does not get exposed to sunlight UV (which wrecks GOOP in a few months with constant exposure).