The Ultima will handle just about any motor you can fit in it, the gearbox is pretty tough. Spares and upgrades are available to order here
http://www.tamiyaparts.co.uk/tamiyaparts/tpx/kyosho.htm and anywhere else that stocks Kyosho. If the parts are still available from Kyosho Europe they will be in that list. The UJs and wheels to go with them are listed there so are still available.