If you want to use that bike, replace the fork with one that has the mount built in and thru axle. There is a reason for thru axle and you found out the hard way.
And while doing that you'll start to find out why you can't just replace that, then your lever won't be right, and then a matching lever for the rear, then the cable, and on and on and on. I did an upgrade to an old MTB to make it a better city bike and I ended up replacing almost every moving component.