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My 1991 Prelude SR-SE 4ws has this motor and i was wondering what could be done to it.
Id like to...
1) Exhaust (full system... header to pipe... preferrably a Greddy system)
2) Intake
3) Short Shifter
4) anything else youd recomend.

Ive looked all over the net for info and specs on the B21A1 and have found nothing. I know its a special canadian engine and that its based off the B20
Please please help me as im in a predicament as to keep the car (as i do love it) and mod it or find something more common. I want to play with it... thou seemly the 21 isnt a player friendly engine :(
Displacement (cc): 2056
Power (bhp@rpm): 140@5800
Torque (ft/lbs@rpm): 135@5000
Compression Ratio: 9.4:1
Bore (mm): 83
Stroke (mm): 95
Cam Design: dohc
VTEC?: no
OBD: 0
Year(s) Produced: 1990-1991
In the following car(s): Honda Prelude Si

The mods you listed can be custom made,the only one you my have problems with is the short shifter.I'm not sure what other motor yours cross references with but find that out and parts will be easier to find.
modding it anymore than basic bolt ons is going to be a lot of money and a lot of work.

they are the red-headed step child of honda motors, the b20a and b21a are. sucks. the really only thing that is compatible with other b-seires motors is the crank... which really doesn't need to be replaced anyway.
the rod/stroke ratio on it sucks, making top end revs hard to come by safely.... and thats why there's a lack of top end power that honda motor's are 'known' for.

IMO, if you aren't too too attached to the car, you are better off selling it for a car that has a better aftermarket.
if its reliable, and you like the car- keep it. it will be a great daily driver for 200k miles or more. and save up for a cheap hatch or crx for a grand or 2... and start building that as a project car.
lol ok dude :)

and what kind of rods do you plan to run? or are you going to deck plate it to hold the 93 mm stoke? :)
and then get a custome hood so it fits in your bay? :)