Reports are that Scamp
will install one for under $500. Can't verify that for a fact but it seems right. A local trailer store (utility, landscape, horse trailer, etc) can order you an axle and install it for you. I would think custom work would need to be discussed with the technician at a trailer store near you. If you're up for it and you have a cutting torch, you could always take on the project yourself and if you then use a bolt on axle, you might find all this within your own wheel house of capability. The internet is stuffed with photo arrays, video clips and so forth showing dozens of trailer axle replacements and customizations.
The point is, I guess, that there are many ways to skin this cat, and way more trailers that get new torsion axles than just FB campers. Some get shock kits at the same time.