as you said, that is a vintage gsp, a very accurate firearm, which got many medals in its time.
I would have opted for sesrching for the propper magazine, to keep it as original as possible,but thinking again, those pistols were almst nevr kept as they cam out of the factory. Match shooters used to modify anything they don't like in their guns, and in the end each one of them ended up with "his" gun.
Function, not looks, is what drives competitive shooters all over the world.
Maybe you don't know or maybe even you are not interested, but in my time in international shooting, GSP ruled the arena of Rimfire and Centerfire pistol, except for the ladies sport pistol, where some women disliked their nose-heavy feeling and went for the older Hammerly 208.
And in the olympic Rapid fire match, the only gun you could see for yeasrs was the famed Walther OSP. Again, almost everyone was extensively modified.
I still keep both my GSP's 22 and 32, along with my OSP, some medals and a national shooting team jacket dry cleaned (that's the one I'll wear in my coffin and don't like to look raggy)
Congratulations and enjoy, shooting a GSP for fun is like driving a F1 car for groceries. ;-)