If you get the battery cover off, the next challenge is finding a mercury oxide battery, since they were discontinued due to environmental concerns. Mercury oxide batteries provided a fixed 1.35v that was unwavering, not like the alkaline batteries we use today, so do not try to substitute with alkaline!

It is possible to get Wein air cell batteries, which use hearing aid battery technology and are activated by pulling an airtight seal off the battery. Once activated, you use it up or it wears out...there is no resealing to put the battery into suspended animation. Some places like zuiko.com sell an adapter that has the form factor of the PX-625 mercury oxide, but which accept smaller silver oxide cell, and outputs a fixed 1.35v that would be needed from a PX-625 mercury oxide substitute.