Done that already
BTW, any autopilot is vulnerable to a failure of the pitch or roll source - whatever it is. A TC can fail, just like an AI can fail. The KI256 just happens to be an expensive bit of kit because it contains a flight director (and because Honeywell are milking it).
There is an exact electrical replacement for a KI256, but electrically driven so not vulnerable to a vac pump failure, from Castleberry in the USA, which is TSOd, but you would need to apply for a field approval (N-reg). And you would need a separate power source for it, obviously (a second alternator, or a sizeable backup battery).