Looks to me like your brackets are fine, but the nuts are locked into the threads.
Ill assume they aren't cross threaded and the threads are still intact...
But if they are, some have bought threaded rod and made their own replacements
Have you tried PB Blaster on the nuts?
It's a super penetrating oil we all use to get things apart.
As a last thought, you can apply a torch to the nuts for a few seconds.
When you put it all back together, use a dab of antisieze compound to prevent future problems.
edit: some motor manufacturers have replacement hardware.
What brand is yours?
You can also cut the nuts in 1/2 by cutting along the axis of the thread down to the thread from opposite sides of the nut. Get (a) new nut(s) and apply anti-sieze as per Anole. This is a common problem with stainless steel threads.
If they are stainless they won't be corroded in place. They'd likely be galled. I'd put some lubricant/penetrant on like PB Blaster then take them off with a box end wrench or deep socket. You want good nut engagement so they won't round off, then turn until they either turn or twist off the rod..
Thread galling seems to be the most prevalent with fasteners made of stainless steel, aluminum, titanium, and other alloys which self-generate an oxide surface film for corrosion protection. During fastener tightening, as pressure builds between the contacting and sliding thread surfaces, protective oxides are broken, possibly wiped off, and interface metal high points shear or lock together. This cumulative clogging-shearing-locking action causes increasing adhesion. In the extreme, galling leads to seizing - the actual freezing together of the threads. If tightening is continued, the fastener can be twisted off or its threads ripped out.