A friend of mine gave me a beautiful OM-1 with some lenses. The only caveat is that I don't have any batteries for it. The camera takes mercury batteries. I was wondering if there are any options to replacing it or modifying it so I can take advantage of the built-in light meter.
I just dug out my old OM-1 recently and had the same problem. Here's the cheapest solution. I had some AG13 batteries I bought for VERY cheap a couple of years ago; 100 of them for not much, and I kept them in the fridge. Two days ago I put one of them in the OM-1 and folded a small bit of aluminum foil and stuffed it in on top of the battery and screwed on the cap. The meter worked!
Next, I did a test. Using the same target, at ISO-200 and 1/30th sec, the OM-1 meter centered at f 5.6.
Using the same settings and target, my Olympus OM-D E-M5 told me f 4.5. So about 1/2 f-stop difference. Not too shabby for 40 years' difference.
The AG13 is a very common item. Google "AG13 equivalent" and you'll find Energizer 357 and Duracell A76 and many more. Just drop in the battery flat side up, and fold a small piece of foil to take up the gap between the battery and the screw-cap, tight and flat enough that it can't shift and short out.
Then get a bulk order of AG13's like I did for MUCH cheaper. Maybe a few go bad and turn green around the edges after a year or two in the fridge. So what? Toss the ones that look bad, and get a decent battery tester that tests button batteries, and pack a few extras on a trip.
Surely you have a cheap or better modern digital camera you can use for similar tests? Try a bunch of the cheap batteries against tests on the same target with the digital, and look at the EXIF data compared to what works in the OM-1 without using film. Basically, the cheapo digital is working as an external light meter. Do that till you get confidence in your batteries before you use them on relatively expensive film, with it's attendant developing and printing costs. And may the Farce Bewitch you, aka good luck.