My elantra has no strength

It turns out that the car changed the engine because the other was decompressed, it started well and everything but as it was walking like something was being covered and that was gradually removing the strength, until I ended up wetting the spark plugs, I thought it could be the Catalitical that could be covered, but disconnect the escape forward and still fails me , do not it be a computer? Buy new spark plugs and improve a little but the same after the strength is taken away, it seems that it is a problem of air entry or something, because I have to disconnect the valve that it has in the valve cover to regulate me , please someone who can help me, I would appreciate it very much ....