Another great and very cheap option for mercury battery replacement is the 675 hearing aid battery. These are virtually the same as Wein Cell rated at around 1.35-1.4V but cost only a fraction of the price — here in Poland 40-50x cheaper, since Wein Cell costs around 50 PLN, and you can get 6 pack of 675 for like 6-8 PLN.
You can put them (like any other LR/SR44) into a cheap adapter without a resistor or use a rubber oring if your battery compartment is shaped to fit an old PX battery. I use 675 with Canonet, Pen FT and OM-1 and work great, giving me very accurate readings.
Robert Pustułka