Its possible.
The hardest part will be running wires and what not. The typically the cruise control amplifier sits under the dash. It makes connections with the following:
(and this is true for all of the cars equipped with the 14-pin cruise control)
1) Power/Ground
2) Actuator in engine bay
3) stalk on steering column
4) brake pedal switch
5) vehicle speed sensor at back of cluster (different for 126)
The 124's cruise control harness might not be long enough in needed areas. So might need to lengthen it..other than that..i dont think its impossible.
Its do-able however provided there is time, parts and patience.
It seems that the 123's mount hat one linkage assembly higher up/closer to the radiator than the 126's so you might need a shorter rod to go between that and the blue rectangle piece.
It looks like you'll need to send her the actuator/actuator mount and one piece of linkage:
As for a cruise amplifier, i think the applicable part numbers would be
001 545 79 32
002 545 32 32
002 545 91 32