Two ways that I know of. The easiest is to get ahold of Dr Honda and order a set of 14mm hexes. They work great. The other is to do the Swift Buggy conversion. The new hexes are the easiest, but you may start breaking shafts or other parts due to extra strain. The Swift Buggy conversion is more expensive and more involved, but probably the best option. Here are a couple links for you:

Swift Buggy conversion: (Post #32 by Rolland)
http://www.rcuniverse.com/forum/m_38..._2/key_/tm.htm

14mm Hex discussion:
http://www.rcuniverse.com/forum/m_3532034/tm.htm