I see that this is a year old, but just wanted to share in case others are still looking up the topic.
Today I replaced a Shimano Selecta with a new, cartridge style Shimano BB (note: the bike is a late 70's or early 80's Apollo Custom Sport road bike).
The weirdest part was removing the old one, but came down to three steps:
Remove both crank arms with 6mm Allen wrench (they pull themselves off, no crank extractor required!) and remove the chainring by hand.
Remove the lock nut on the non-drive side with a 23mm wrench. (I think it was reverse-threaded), then remove washers, bearing rings, cone, until the axle pulls out of drive-side.
At this point there's just the "cups" left, they kind of dish out the end of the BB shell, and then curl inside where they have a splined inside. Using the old Shimano freewheel tool (I think it's called FW30) remove the 2 cups separately (remember which side is reverse threaded!) and then give it a good swipe inside with a rag.
At this point, I went to the shop and bought a Shimano cartridge 68mm x 117 (BB shell width x spindle length) and threw it on with some grease on the threads. Of course, the spindle length will be determined by your crankset (they usually have a specific one that they're designed for) and the cranks from the Selecta will not fit, so if you went for the cheap, simple,
A few hours of tinkering, and then $15 for the BB and another $5 for a crankset from your local community exchange. I think the end result is worth it, instead of having to switch to a one-piece crank, as it leaves you with more options for future uses of the bike (double, triple chainring, SS, etc).
Good luck to all future Selecta removers. Your struggle is not in vain!