I set one of these up for a bassist in one of my drummer's other bands and achieved the lowest action on any bass I've ever done. The quality of the neck was quite superb! Glwts!
Probably electrolytic capacitors on the preamp power supply not working correctly. If they are largeish, sometimes the vibration can fracture the solder around one of the pins too.
Looking at it, I think you could fit a 32" 20 fret neck and move the bridge back 3/4" and have it intonate ok with the travel on those saddles. I know you said you weren't keen, but it's doable, if you can source a neck!
I'm looking for a medium scale fretless neck that I can use as a 24fret short scale for my current fretless. It should work, as my bass has an unusual neck attachment.
I would think that's about right for a bespoke neck. Unfortunately
From the photo, the strings don't seem remotely aligned with the pole pieces. Maybe a tiny neo magnetic on just the e pole would help? A similar sized pickup with blade/ rail configuration might just pick the outer strings better?