Regarding pictures of original interior... The "Site Search/Google" feature (bottom of the search menu in the toolbar) is the best way to find stuff on this site. If you look at enough Cadets, you'll get a sense of what's original and what's not. Of course you can do a general internet search as well.
In the end there's no compelling reason to return the trailer to original condition. It's uncommon, but not so rare or special as warrant preservation. Do what works for you. Thin-walled fiberglass shells do depend on interior cabinetry for support, so a totally open layout is not a good idea. You'll want to incorporate at some floor-to-ceiling components. In my Scamp
that's provided by a full-height closet by the door (a very common feature in many fiberglass models) and a metal support between the lower and upper galley cabinets on the street side.
Regarding original hardware... Some things are no longer made. Locksets for the doors are often a case in point. A locksmith may be able to salvage what you have with new parts. Otherwise you'll have to purchase something current and make it work. Fortunately fiberglass is a very forgiving medium to work with. You can cut and add and shape as needed to accommodate different hardware.
Some parts may still be available through European suppliers, but you'd have to ask other Cadet
owners for details. Of course it won't be cheap to get them here.
Best wishes on your renovations!