I recall putting a standard battery in my canonet. It worked, but the meter was off by some amount. I want to say that I had to set the camera for 200 when I used 400 film, but I don't remember the specifics.
This could actually be an advantage. The camera can only be set as high as 800. With the "wrong" battery, you could use something Delta3200 at 1600 and still meter correctly. Again, check the meter with the battery.
Use the mercury battery issue to talk the price down at the pawn shop. Also, check the rangefinder, they can be a bit dim (just like me!). If so, talk them down some more, then clean it up yourself.