I have just had a Yashica Mat 124G serviced by Camera Check Point at Dubbo and they have installed a Varta 625 U alkaline 1.5V replacement battery.
They noted on the service docs that the meter calibration was checked as OK with 1.5V battery. Tests against a Canon T90 and a Gossen meter seem to bear this out,(there is about a one third stop spread of the meter reading between them all).
There was no mention on the service docs of any circuitry mods to cater for the 1.5V battery so it would appear that my model is happy with the modern batteries. You may like to try this battery and compare meter readings for yourself.