Well, you have 2 options.
Option 1 - Get a water analysis from a place like Ward Labs. It will tell you everything you need to know for brewing.
Option 2 - Start building your own water with distilled/RO water and salts. With Mg that high, you are already going to have to dilute with distilled water 5:1 of your water.
With that said, I recommend option 2. My water is awful, like most Ohioans, so I choose to build it from scratch.