Added. Thanks.That's an F3 1994 bike, search for PACE serial database on here and get it added .
Measure the other one and you should find some online.
And call it a Brake Boss
That was me using a pair of pliers to grip it trying to undo it. .Looking at the mangled threads on the brake stud it looks like it has either pulled the aluminium thread out with it or certainly done some damage on the way out. I'd check the thread in the frame with a bolt of the same diameter and pitch, might need a repair with a helicoil or going oversize / drill through with a threaded insert.
Plenty of elbow grease with Brasso is good for a polish or Autosol if marks are a bit more stubborn. These are lovely bikes to ride and well worth the effort to fettle it. Hope you get it sorted and out for a ride on it soon.
Legend! Thanks.https://www.sourcebmx.com/products/sunday-removable-990-brake-mount?
Threaded portion might not be as long and might need a little counter bore to sink it flush but should work fine if the threads in the frame are good. If frame threads are bad then then a helicoil will sort it.
As mentioned before I'd check the frame threads first