yes, if you want to add a midi through socket, you need to install an additional buffer and two resistors.
Have a look at this drawing, which I stole from :
Here two inverters are used instead of a buffer. The input of the buffer/inverters are connected to the output pin of the optocoupler. In Case of the Juno-66, this is Pin 5 of the CNY-17 on the underside of the board. The part is pretty large (2.54mm pitch), so soldering a wire to it should be relatively simple.
I have not come accross a Kenton install so far, but I am assuming you can remove it and have the Juno back in the original condition. This means installing the Juno-66 is not a problem.
If you are unsure, post some pictures here (Kenton do not publish installation manuals, so it is impossible to tell if modifications were performed during the installation.
 http://whitefiles.org/b1_s/1_free_guide … s/h10d.htm