With a few exceptions, foam surrounds were pretty much the norm for woofers in the '70s and into the '80s. They worked very well, but they oxidize and rot out in 10-20 years depending on conditions.
Definitely get the foam surrounds replaced. It's not a big deal and there are plenty of places that will do it professionally. Any Dahlquist DQ10s and 1970's EPIs and Advents in operation today have had it done. I once bought a pair of small 20-yr-old EPI bookshelf speakers for $10. As I was loading them into my car the woofer's foam surrounds crumbled in my hands. I ordered a repair kit from a vendor ad in the back of Audio Magazine (it was a long time ago). I repaired them myself for $15 and about 2-3 hrs time.
However, that was a 6" woofer and the JBL uses 12". As you go up in cone diameter, the more difficult it is to keep the voice coil aligned while the glue dries. One method is to remove the dust cap, wedge some toothpicks into the front of the gap to keep it centered, glue the surround and let it dry, then remove the picks and re-attach the dust cap.
But if that's too much trouble, hire a pro. Even if you pay $100 and you end up not liking the speakers, you can sell them for current market value, which beats throwing them away.
