Maybe some one will have one to sell since many on here install PP and have a spare.
I would first check the line, ballast and pitot. The ballast are mounted on the gas tank. If you remove the rear seat you can see them.
If you are looking to trouble shoot, just swap the lines going to the back of the speedos on the dash. That will help you figure out if it is the tubes or the speedo.
I seem to recall a company that restores the old airguides too. Maybe someone knows of a website.
1996 Prostar 205 LT-1