I have been Repairing and Reconing JBL Musical Instrument and Professional Products as well as about any other speakers that you can think of for over 30 years. For a great deal of that time (the Waldom years) I would not use anything other than JBL parts in a JBL speaker. This is just not the case these days. There are reasons the D-120 & K-120 sound the way that they do and the current JBL recone kit is not made with any of the parts that created that sound or where originally used in that speaker.
The C8RE120 (8 ohm recone kit) is what is available now. It will fit, and will work just fine, but will sound nothing like the original D-120. Not that that is necessarily and bad thing, some players may like it better. The reasons for this are many and fairly detailed so I won't go into that at this point. What I will say is that the retail price for this kit in the most recent JBL Transducer Parts List is $264 and that does not include any shipping or labor. Now I have always been and will always be a JBL speaker fan, but that is ridiculous.
There are some very good aftermarket parts available for these speakers today and most are closer to the original parts in every respect. Reconed properly these vintage JBL's can sing again. But parts alone a good recone do not make. The technique used in the recone process is every bit as important as the parts used. Also without the proper Fixtures, Shims and Adhesives the outcome could be disastrous.
The videos linked here give a good overall idea of how a recone is done but leave out some Very Important Technique's. Remember. Just because the parts fit don't mean the are the correct ones for the job.