Using old cameras have their weaknesses. It is hard to find the batteries that suited them. This is how I improvised with my Yashica. I found this solution from youtube. I don't remember who posted the video but thanked to the uploader I can use my Yashica now. I want to share with you so that you can use your Yashica MG-1 too. They might be other ways but this is the method I used.
This is the battery for the Yashica MG-1. The HM-4N. Mercury battery. You can't find this 5.6v battery anymore.
You have to use two batteries instead. The 123 size and the A76. They are 3v and 1.5v respectively. The Volt would be 4.5v now but it is enough for the camera.
Make sure the negative side is put in first for the 123 and then the A76 negative side on the positive side of the 123.
I put a spring from and old small flashlight to ensure the batteries doesn't moved inside the battery tube. You don't have to do this if you don't want to. they will still work.
p/s. was thinking of making a video but I was too shy. haha.