My personal (amateur) recommendation is to use a zinc cell.
If you want what a "preofessional who markets thier skill at a price" take heed of the recommendation above for a Shottky.
If that is not enough I'd suggest trying that micro 1.3V regulator, TPS79913DDCR, as it cost only about a buck.Quote from Chamtech photo services website:
BTW, my OM-1/1N (Pen FT, 35RC, 35RD, 35SP etc.) overhaul builds equivalent MR9 schotkey diode into the meter circuit.
If you want to tread new ground and re-calibrate the resistors yourself, here is the OM-1 meter schematic. Resistors R1 and R4 control the LOW end and resistors R2 and R3 control the BRIGHT end of the response.