From the Scamp
factory, the spare is attached directly to the shell using three bolts. Plywood reinforcement fiberglassed to the inside of the shell carries the weight
. I don't know exactly how it all goes together, since the inside is hidden under insulation and rat fur, but I assume the bolts go through plywood and fiberglass. Whether you could restore the original mount might depend on the condition of that plywood. Are there any remnants of that set-up on your trailer?
I don't know if there is another way to mount a spare. The tongue is pretty full, and the bumper pretty flimsy. You might do a Google site search for "spare tire mount" and see if there are any ideas on old threads.