I don't know who made the Magna's for Sunn. I'd recommend you copy down all the numbers on the speakers and contact a local recone shop. They might be able to tell what company made the speakers and if replacement cone, spider and voice coils are available and the cost to re-build your speakers. Then you can evaluate a re-build vs. new speakers.
Re-coning speakers is not a do-it-yourself project. There are way too may variables.....correctly prepping the old basket, using the correct adhesives, centering the voice coil, not to mention finding correct parts that will fit. I've had lengthy conversations with the guy that recones my speakers (he's been doing it for over 35 years) and there are a bunch of variables involved that require experience and judgement to select. This is especially true when NOS parts are unavailable and new production parts need to be adapted to an old basket.
Some work is best left to the pros. (my $0.02 worth, no affiliation, etc.)
Ryan