Have you checked the pilot jet? The CA RM250's came jetted horribly rich, with a 52 pilot and 168 main.
With pump gas, I ran 158M w/ a 48P, #2 clip position on the stock needle @ 60 to 85 degrees. And, a 160M, 48P, #3 clip on the 1/2 step (next richer) needle. With pump gas, the bike needs to be jetted carefully or you will get nasty pinging. The 1/2 step needle seemed to help a bunch.
You may want to try race gas. With Arena Cross, you won't be using much fuel anyway. The guys who used race gas in their 01 RM250's reported significantly better performance. I rode a bud's 01 250 w/ R gas, and it was a totally different bike than mine; smooooooth powerband w/ more power everywhere.
Also, consider a different pipe than stock. I ran the Bills Pipe, and it added power everywhere and extended the powerband over-rev considerably.
Another cure, would be get your hands on a 02's powervalve and/or cylinder. The 02 RIPS! I'm not sure what parts you would need, or what will make a difference, but I'm working on finding out.