I would think your most cost effective repair may well be to get the manufacturer's trained service tech to check it out, these units have pretty good computer diagnostics if you/they have the program and cables and training.
my WAG [the 'w' stands for wild, the 'a' refers to a name for a donkey, and the 'g' for guess] is the directional control in the 'tiller' control handle head, but like you said, these are kind of 'high dollar' parts to swap parts, and you would still most likely need to get the "tech with the laptop" in to setup the new control cards.