You can run a .21 with [link=http://www.neweramodels.com/cgi-bin/item.cgi?session_id=mbpomquyofkdoiayoncspsihpfcoji ig&part_id=1816]this[/link] kit. I think you can fit a .18 in without any mods, but don't quote me on that. Of course you could always go with a high HP .15 racing engine. Or just buy a second .16 from Cen, they are pretty cheap.