I'm not sure about that Skamper in particular, but external lift arms were standard on many, many p'ups and most probably, those are original to that camper.  Whether they're original, or replacements themselves, you can get more at http://www.popupparts.com.  The specific page for lift parts is here. 

I doubt you'll find an entire roof for sale anywhere, but many folks have built their own.  It's a common issue with all makes and models that have water damage.  Some folks take pains to replace the roof exactly as it was originally built, even using the original covering in many cases.  Others build an entirely new assembly with different materials.  I've seen builds going by on the various forums made from structural metal with sheet aluminum to fiberglass over plywood and all sorts of innovations in between. 

The ad you linked didn't say anything about it needing a new roof; have you talked to the owner?  To be as old as it is, it really doesn't look bad at all.  Unless you know something specific that you haven't shared with us, you might find that the roof is just fine like it is.

The two most popular places to have canvas made are Bear Creek Canvas and Canvas Replacements.