You can change both the pinion and the spur if you wish (AE Kimbrough Spurs are 100% compatible - here is a link to a 81t spur as an example:
http://www3.towerhobbies.com/cgi-bin...?&I=LXEAK2&P=7)
www.gearchart.com is excellent for charting the perfect pinion/spur combo for your motor. I wrote a quick walkthrough to use the site. Let me know if you need this info.
Dan