If you don't care about being original, then you could replace that D130 with something else. The sound will change quite a bit as nothing sounds quite like the D series JBL. If the speaker is bad, you could get it reconed for around $150 max I would guess. Ted Weber at Webervst is the best and the widely acknowledged expert on speaker reconing and has very fair prices too, but he recently stopped reconing speakers other than his own brand and recommended a couple places in California to do it instead of him. If you want another speaker in there instead of the D130, but you want it to have the same apparent volume level, then you need to get something with the same impedance rating, and the same level of efficiency. Something around 100db and 8 ohms should work fine. I would personally recommend to get a Weber California type speaker, which is around $170 or so I believe. They are his imitation of a JBL sound and sound reasonably close. You could ask Mr. Weber himself some of these questions by emailing him or posting something on his bbs. Check it out at
www.webervst.com.
If it was up to me, I would get the speaker reconed, if that is what is wrong with it, and put it back in the cabinet. JBL D series speakers sound very nice and are worth a bit of money nowadays too. You can find them on ebay but prices tend to climb quickly. If you need more volume, I would either get a second cabinet, or get a different amp with more power, or mic it through a PA once you have your tone as you like it.