a DOHC ZC would be the easiest swap that would be worth it. next would be the LS, pretty much just as easy as the ZC... but you're gonna need mounts. then would come the B16 this is just a 'lil bit harder cuz you're gonna need to run a few wires for vtec and get some mounts. those are the three most common swaps into 4th gen civics and 2nd gen crxs. out of them, i would probably choose the LS. it is very boost friendly and is not very hard to get up to par with the b16 hp wise, and would still have more torque. but if you want some serious all motor, that the b16 would be a better choice than the LS. the ZC is really just a compromise of cost... as it is cheeper than the b16 or the LS. the fact of which would be better (LS or b16) could be argued until the end of time, so just take to note that what i have said is just my personal oponion and you may hear different from others. good luck.