Could you post a few more pictures? Some a bit wider?
And for sure a couple of pictures of the warped plate.
From what it looks like to me it may not be a crack due to weight, the odd bend near the crack would suggest some sort of trauma that caused the crack. It looks like some sort of torsion pulled the metal down and broke it. This is what I think due to the fact that the plate that the C channel is attached to is much thicker than the C channel and is supporting it so it would be an unlikely place to see a crack, but I'm certainly not 100% sure.
That would be a good thing, since the worries about the frame being structurally unsound wouldn't apply. But since you mentioned that the plate was warped I'm not sure on this...
The only way I could see this being a break due to something other than trauma is if the trailer was wrenched from side to side and bent sideways causing that metal to crimp in on itself and then break...possible... are there issues on the other side that would match this?
This can certainly be fixed.
I just did a big frame reconstruction and I assume the C channel is the around 1/8th in on yours as well.
It would need to be bent back to butt up against each other and (if it is just a trauma break) I would just butt welded it and call it good (just what I would feel comfortable with IF that was the case).
You could certainly add additional steel if you thought it needed it.
The big question in my mind would be; why is the axle plate warped? Rust can get between the plate and the channel and bulge it out, is that what is happening? If so you'll want to decide how involved you want to get with this issue...I had the same bulging rust issue and opted to cut out the parts of the plate and replace them with new plate.
Here are some pictures of the frame work I just finished.