ive been researching this as well. I have a 90 S-lude and the only thing that ive found out as far as the b21 goes is to swap over a b20a5 head. Itll give you a significant bump in compression because of the smaller dome and it bolts straight up. Unfortunately, the b20a's and B21's are kind of the black sheep of the b-series family.. any other B parts are just not compatible. the closest relatives are H and F series.. I HAVE heard of a B16 head swap but it was completely unreliable and had no usable power.
Heres a list of interchangeable parts..
B-Series Cam Gears
B20b Cams (with mods)
b18b Cams (with mods)
Rods from H23
Pistons from B-Series Motor (requires maching wristpin area 1mm larger)
94-01 ITR stock LSD fits the 90-91 tranny with mods.
H22 dual valve springs
edit:also an F22 throttle body. swaps right over.
hopefully this was somewhat helpful. these are just cost effective suggestions, if you have the cash yank the b21 and drop in an h22. it is by far the easiest to swap in and is well worth it (heres a link on one very etailed write up I found "http://www.preludepower.com/forums/showthread.php?t=190047") hit me up if you have any more questions!