The reason the frame is weak is thin material on the early frames like the early Scamps and the pre failed bends on both sides.
If you look at the frame rail tubes you will see that they are already buckled and the inside is forced inward and the top and bottom have slight bulges.
In my opinion this area should be (or should have been) plated over these areas as most of the strength of the tubs has already been lost.
A fish plate on the inside and outside, tapered on both ends to reduce the stress riser generated by the stiffer area and you should be good to go.
It is likely that there are already cracks if the trailer has been used hard over rough roads. My experience has been with Scamps, but the frame and basic trailer is really a copy of the Boler