I would say the best improvement would be to do the roving special for all the MAGIC CITY lights/Targets.
Its been a while since I played one but I am assuming the rollovers and targets are initially lit, while the name spell lights are off. And, as you get a letter, the rollover/target goes out and the associated lamp in the name lights up. If that is correct, I would also want to incorporate the lighting of the letter that would award special, not just the spotting arrow. In fact, now that spotting is over, I might also want to turn the arrow off.
Obviously, the spot arrow runs off the Match unit. So, you would have to look and see if you can find 10 unused rivets in a row that could be used to control the roving lamp. If so, you would add a switch to the special when lit relay to put power to the wiper and then have to run 10 wires to power the playfield letter lamps. Hopefully, you could pick those up at the trip bank (I assume there is a trip bank with a trip relay for each letter and star) where the switch goes from turning the roll over / target lamp off and feeds the name spell lamp instead. This should reduce wire length.
If the stepper for the match is up in the head (probably is for the bell striker), it sucks since you would need to run at least 10 wires down to the playfield where the trip bank or lamp sockets are. If it were on the playfield, it would be a lot easier but then they would have had to run 10 wires up to the head for the match lights. Right now, I cannot think of how you could re-use the arrow lights to power the roving letter lights without the use of 10 make/break switches fed via the special feature.
Of course, I may have guessed wrong on the spot arrow stepper being part of the match stepper. If I did, then it would be a much easier implementation. The easy way would be to do just as you suggested and just re-purpose the spot arrow as the special target indicator but Gottlieb certainly would have not done it that way (I don't think?).