Rather than modify, the best thing is to ensure the engine runs as it did from the factory. Both of my 122S's (when I had them) had good camshafts, a C cam in the later one, and of course rebuilt carbs that could be leaned and still give good driveability and idle. Also important is to confirm that the distributor is advancing as it should. Both cars could get 35-36 mpg Imperial on the highway without overdrive. (I've never like overdrive) That would mean about 29-30 mpg US gallon. That's quite good for an old car.
Of course good radial tires, don't pack around the big tool box, and keep the timing advanced to barely avoid pinging, and you'll get very acceptable mileage.
Just what all that would cost I don't know, but something well south of $2k.
Rhys