Could be a clogged cat, pull out the ego sensor but leave it plugged in. Best way is to get a tool that plugs in place of the ego with a pressure guage, rev up should never go above 1psi, 6-20 psi is really bad.
Another way to tell which requires a bit of good judgment and experience, is to leave it unplugged and rev it up. If you feel a bit of air come out it might be ok, if it comes out at the rate you would feel it come out the tail pipe, definitly a problem. Or if you can find something rubber and flat to cover it with, rev it up and it shouldn't push it off.
And yet another way would be to measure manifold vaccum, which is probably a test you would find in the book for backpressure. If it has a hard time pushing air out the exhaust, it will have a hard time pulling air into the cylenders, which will lower your INTAKE manifold vaccum. The book will probably have manifold vaccum specs, however poor manifold vaccum could also mean your timing belt is stretched or skipped a tooth, which could have the same performance symptom. Thats why I like putting something rubber and flat over the EGO sensor port to actually determine quickly if the Cat is plugged or not. If not it may be a timing belt problem.
As for a motor swap, I would recommend a JDM sohc zc vtec. If you live in California I would recommend a d16z6 (same with less compression, easier to pass smog/certify). Both of these motors will drop right in, minimal to no modification. In fact you could probably get a complete engine/tranny/harness/ecu from canada for $800. d16z6 is approx 120hp, the JDM version ZC sohc vtec 1.6 is usually approx 140hp at wheels. These motors are great because you can slap a basic turbo kit on it to reach 190-200 easily, or go all out crazy with turbo technology and get 400.
Naturally asperated are not quite as powerfull or sound as good as a b16 or b18, but with turbo will put out more power than your car can handle anyways. b20s h22s, too much torque and modification to be realistic. Don't go for those unless you like to make a mess of your car or crash it from excessive torque affecting your steering. Not to mention if you live in California, your chances of ever registering your car with any B or H series motor are pretty slim as referees usualy dont like it.
ZC motors in california however are now LEGAL in cali, but very difficult to pass smog. May end up having to wrap your exhaust manifold and get an obd2 cat like I did.