What engine can I swap in a 89 nissan sentra b12

Whatever you can fit in there, Scooter.

I know a GA16DE (the 1.6 liter engine from a 1991-94 Sentra or 1995-99 200SX) will fit in there without having to do too many modifications as far as motor and transmission mounts go (did one with a friend a few years back)....but if fabricating (or having someone that does fabs) and budget aren't an issue the possibilities are pretty wide...you can go SR20DE from any Sentra/200SX SE-R, SR20DET from Bluebird, Avenir or Pulsar GTi- R, SR16VE from the Pulsar Serie VZ-R, SR20VE from Nissan Primera (the Japanese counterpart to the Infiniti G20) or a QR25DE from any 2002-06 Altima or Sentra SE-R/SE-R Spec V. Or even a VQ35DE 3.5 V6 from Altima, Maxima or Murano. It all depends on how much you wanna spend in all that's required for a swapto ork right.