Well FFH351, I was under the impression you used a new SE-R motor (but I could have just assumed that)?
But, yes, to original poster. If I were in the position to milk some performance out of a '91 Sentra I would track down an SR20DE motor/tranny combo out of a '91-94 (pretty sure on that year spread) Sentra SE-R. These were very potent little sleeper cars and the SR20DE is a very durable and proven motor (you may have heard of the SR20DET motor from the Nissan Silvia, this is a turbocharged variant of the motor found in the SE-R... and is very popular with the 240SX guys and the Datsun 510 guys). You could also track down an SR20DET motor to drop into the Sentra, but I recall reading somewhere that the gearbox wasn't quite up to the torque load from the turbo motor. But these are by no means written law, I just recall reading this in an article at one point in time.
Build 'em Light and Wind 'em Tight.