I would suspect that if you could actually find used Series I guts for sale, that any competent luthier should be able to install them relatively easy (and for that matter, cheaply). The problem would be that you are most likely going to be waiting for several years (if not more) to ever see them for sale. In my humble opinion, your time and money would be better spent on an SF-2, if you don't already own one. It's pretty much the same as buying Series II electronics in a separate unit, and they hold good resale value if you are inclined. One of these combined with on-board Signature electronics would provide an insane amount of flexibility. There would still be the difference in pickups (single coils vs stacked) and the dummy coil, but the SF-2 really is a swiss army knife for tone.
