Being that what the other guys are saying is right, I won't kick your Kustoms.
I would suggest if you are hell bent to do it, just buy Carvin drivers and put them in. I'd also get the Carvin two way crossovers as well as replace the HF driver. This will not improve the quality of sound, but you will be able to make it the same only a little louder. The box is as critical as the drivers. A fuzzy carpeted box is a fuzzy carpeted box, even though it might be covered in tolex. I wouldn't waste chasing down any old JBL drivers or EV drivers, just throw Carvins in. Better drivers does not necessarily fix a mediocre design.