I had one of these a while back and when I measured the pickups, a replacement set would never fit "just right". EMG 35 size, I believe, were the closest. (Bart M3 would be same size, $100 each at bestbassgear.com)
So if you do plan on a swap, some cosmetic fixes may be in order. Pickup rings, maybe some wood shims painted to match, and may even have to widen the pickup cavities to fit something in.
HOWEVER... what I did with mine to rid it of single coil buzz was to switch the push/push pots with push/pull and wire the pickups series/parallel. It gave a lot of tonal options and never had single coil hum.
Didn't cost much and worked great!
And if you go with 3 push/pull pots, wire it volume/volume/tone, you can have each pickup series/parallel, but also add series/parallel to BOTH pickups and have even more passive tone options.
Without adding a battery cavity, it's true that there's really no space to put a preamp in there. But the Push/Pull pots fit in no problem.