You can upgrade to either zeniths or webers.
The originals are rebuildable. The kit is out there.
Granted, the work hasn't come up recently here, but check the archives here and at
http://www.mercedes-veterans.com. Pretty much all you'll need docs-wise is the "starting 1959..." service manual. Go for a paperback copy. There's one on ebay as we speak.
-CTH