there's lots of options - all depends on your budget (someone check on the obd requirements as I can't remember them correctly)

Cheap Upgrades (change engine but attaches up to the stock si transmission)
just replace the d16a6 engine - it has good power and uses the ecu
dohc zc swap 130hp, requires new ecu
d16z6 sohc vtec 130hp - requires new ecu and obd 1 conversion
d15b sohc vtec 135hp - requires new ecu and obd 1 conversion

More Expensive upgrades - have to get motor mounts to put these in
b16a1 or b17a (from 90-91 integra gsr) sohc vtec 160hp - requires b series cable transmission and ecu
b16a2 dohc vtec 160hp - requires b series cable transmission, ecu and obd 1 or higher conversion
b16b dohc vtec 170hp Civic Type R - same as b16a2

b18b - dohc non-vtec
b18c1 - dohc vtec integra gsr motor
b18c5 dohc vtec (ITR)

Even more expensive - have to make clearance
b20 - from cr-v non-vtec
crvtec motor (b20 block with b16/b18 dohc vtec head)

k20, k24

there's even been a v8 stuffed in a rex before...some guy has a crx with a nsx engine in the rear...