I damaged the tongue portion on our 16ft Scamp
frame on a recent trip to the mid-west. I was "asleep at the switch" and didn't slow down enough at a railroad crossing in Nebraska.
After limping into a small town, I bought a couple of landscape timbers (the most warped ones in the pile) and some ratcheting tie-downs to make a temporary sub-frame. This worked fine for the rest of the trip.
Since I have a friend who is a welder by profession I bought steel reinforcement and cut and fit everything beforehand. I also cut away the fiberglass and flooring where welding was to be done. A couple of hours and all welding was completed. Along with this we inspected the rest of the frame and added gussets in several places where things looked weak.
I would think you could do something similar. Since your original frame has lasted as long as it has already, I would think it would last a lot longer. Perhaps all you need is some reinforcement here and there and it will last you out and be a lot less work.