Fairlight-CMI

I have recently acquired Peter Connelly's Series III, and, as he 
mentioned, the plastic at the end of the G-Pen chipped and won't hold 
the tip in.  Does anyone have any ideas for fixing this?  I have tried 
rebuilding the sheath with plastic, but it won't hold the tip in place. 
Any suggestions would be appreciated.  Alternatively, does anyone know 
of a company that sells compatible replacements for the G-Pen. Seems 
like something any cad/cad system might use.  Thanks.