If you have access to a metal lathe (drilling by hand works too, you just have to be more accurate), you can drill the entire stem out. Once the stem is out, pick up a new metal stem from an auto parts or tire store and you can put it into the cap the same way it is put on a wheel. I did this on my R and it works great for a fraction of the cost of a new cap.
Current Ride (Purchased new in Oct. 84) - 1985 Honda ATC 250R w/ ESR 310cc big bore kit
Wife's Ride - 2009 Polaris RZR 800
Kid's Rides - 2009 Yamaha Raptor 700, 2008 Yamaha Raptor 250
Former Rides - 1975 Honda XR 75, 1979 Yamaha IT 175, 1978 Honda ATC 90, 1980 Honda ATC 110
got sand?