Welcome to Lansing Heritage!
It definitely sounds like you'll need to replace your diaphragm and probably replacing both will be your best bet. With great care you should be able to handle the task, but having it done professionally is never a bad choice.
Unfortunately the correct diaphragms have been NLA for years and both subsequent aluminum replacements from JBL are now NLA too.
I'm not sure what will give you the "best" sound or the best bang for the buck, but here are a couple of alternative choices for you:
https://reconingspeakers.com/product...421-diaphragm/
Good Luck!
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