While I was pretty certain that no flame fired Jensen #50's ever left the "factory" in that configuration, I'm also pretty certain that few if any ever got converted to flame firing by their owners along the way. But this engine proves me wrong on that second count, yet I think it likely that very few, if any others, underwent this "treatment". Boilers perhaps, for other engines and applications, but whole plants including engines and generators, somehow seems unlikely what with the availability of replacement heating elements. Personally, I suspect that this was built this way out of parts, and not from an original and complete #50, so it is still possible that this came out of the Jensen "factory's" back door, so to speak, but that of course is just conjecture.
Now if you are really interested in keeping this unique unit in its current operational configuration, which I'd be tempted to do myself, you might do well to consider converting to a SPS propane burner, instead of that Stuart 504 style meths burner, as I think you'll get a lot more heat that way, which could well be advantageous with that big tank. On the other-hand, if you do wish to return it to a more stock configuration, I would also recommend that you contact SPS, as recent threads have shown him to be a master of Jensen firebox construction, so perhaps if you ask really nicely, and offer up the right incentives .... but please excuse my presumption that such a task might even be considered, though I suspect instructions on the process would be freely forthcoming!
Whatever you decide to do, that is still one amazing find ... generator and all !!!