The inner cables aren't available by themselves, as far as I know, and anyway everything should be replaced as a unit. Smaller diameter cables have less frictional area and glide more easily. Look for the length measurement on the cable sheath and order accordingly; sometimes Hatteras has this info as well. The cost of the cables is not large, but the time involved pulling them through is considerable.

Also, before you go to all this effort, make sure nothing is binding or misadjusted in the control heads and on the engines and gears. Properly setting up cable controls is an art in itself; if you can find a very experienced marine mechanic to help you, it will go better and faster.