This will give you an idea what's possible for a welding project done by someone with very little welding experience.
This is a sheet of ABS plastic that I use to reinforce the inside of one of my panniers. I was reusing it from an earlier system, so I had to patch a bunch of holes in it. All those squares are just bits of ABS that I welded to the plastic using a soldering iron. The holes were backed with the welded on ABS square, then the hole was filled in from the other side with more ABS, melted into the hole and then welded to the ABS sheet.
This is a closer shot of the welds...
Welding works great for repairing damage. This is a shot of my pannier, after I took off all the old mounting hardware from a previous attachment system:
There were a ton of holes in the pannier, all of which needed to be closed off before I attached the new hardware in different positions. I didn't use backing plates on these; the holes were just filled with melted ABS and welded to the pannier from both sides. This is how the rough welds looked:
And how they looked after the welds were sanded down:
This was only the second time I welded plastic; the first time was years ago when I made my tool tube. I bought a cheapie soldering iron from Amazon to use. I'd actually bought a more expensive one that was made for welding plastic (it had a special triangular shaped attachment that was supposed to be for welding), but never ended up using it; the cheapie one worked fine.
Were I in your shoes, I'd try welding that in place. It might not be pretty looking, but no one will see it anyway. I looked the part up on RMATV, and I was surprised that it was only $52.00. There's a lot of stuff attached to it though, according to the parts diagram at Rocky Mountain, and I think you might even have to remove the ABS pump to get the part out. That alone would persuade me to try an on-the-bike repair before I tried replacing it.