If you're close to the ring, your 18-55 may be ok, but something a bit longer as a second lens with a wide apeture and IS would help (around 70-200) as indoor light is never brilliant for sports. I know from the velodrome that I had to use flash most of the time, apart from when all the pro togs were on the finish line flashing away, then it was so bright I didn't use mine. But ask the organisers whether flash can be used. See if you can go to a practice session in the week, just to get your eye in & work out exactly what you need.
Good luck.
