We've broken our frame and we've had it welded without removing the trailer body from the frame. Competent welders will not have a problem.
In our case they put a right angle piece of 1/4 inch steel along one side of the existing box beam. The right angle does not extend up to the floor allowing them to weld the top and bottom of the right angle piece. Once the frame is re-established I would add a fish tale to both ends of the frame. I suggest a fishtail on Scamp's strengthening attempt (the strengthening they do where the frame goes under the
fiberglass shell). While at it I would add right angle steel the entire first 5 feet or at least up to the stair's steel.
Our original break was where
Scamp kinks the beam to form the A shape. This is inherently a weak point and should have a gusset plate.
Our welder had a little spray bottle of water and just sprayed it when thing got warm. Absolutely no problem.