Whell, I think that 20/40 gearing is a must on the jato. It slows down the acceleration to something you can handle alot easyer. To lighten it you should get a pull or roto start, remove the role bar, remove the mud flaps, carbon fiber chassis if you want. For top speed you will want 1/8 foams (you will need 17mm hex adapters). For engines you will want the os 21tm or the sirio S23RE .23 (both have pretty high rpms)
links:
20t cb http://www3.towerhobbies.com/cgi-bin...?&I=LXPWF4&P=7 (this is 10 more than normal cbs but it is the only one that will work)
40t spur http://www3.towerhobbies.com/cgi-bin...&I=LXHFR6&P=ML
pull start http://www3.towerhobbies.com/cgi-bin...?&I=LXDGG4&P=M
roto start http://www3.towerhobbies.com/cgi-bin...?&I=LXEVM9&P=7
cf chassis http://cgi.ebay.com/Traxxas-JATO-BLA...QQcmdZViewItem (I have it and it is awesome)
foam tires http://www3.towerhobbies.com/cgi-bin...?&I=LXKKF5&P=7
17mm hex adapters http://www.neweramodels.com/item.cgi...d&part_id=3244
os 21tm http://www3.towerhobbies.com/cgi-bin...&I=LXRTM1&P=ML
or Sirio S23RE .23 http://www.amainhobbies.com/product_...ducts_id/18963
Oh yea you will also need a new header for those engines http://www3.towerhobbies.com/cgi-bin...?&I=LXFBA0&P=7