check out this engine. http://hobbypeople.net/gallery/146003.asp . its a .18 with rear exaust that will make your maxx a ton faster then the maxx 2.5. you will need to get this kit though. http://hobbypeople.net/gallery/148021.asp this make it so you can use this engine.
i hope this helped. there are other engines that will work but this was what i was going to do with mine before i got a .21 for $81 on ebay.