You might take a look at this site: http://www.tompatterson.com/Trailers/Trailers1.html
There should be some links there to connect you with people who know Silverstreaks.
In general, they appear to be a coach worth trying to save. Unfortunately, even compared to Airstreams they were made in very small numbers, so salvage pieces are going to be hard to find.
As far as making a custom bathroom, you are going to have to deal with some radiused walls. I believe though the walls are straight up and down and fore and aft until they meet the ceiling and at the rear. You should be able to reinstall a toilet, a sink, and even a shower/bath using available RV sources. A suitable waterproof wall covering for shower walls may not be all that easy.
Even though we don't speak Silverstreak here, there are many forum members who have overcome similar challenges, so feel free to ask.