My elantra has no strength

It turns out I swapped the engine in my car because the old one was losing compression. It started fine and everything, but as I drove on, it felt like something was getting clogged, and that gradually reduced its power until it was fouling the spark plugs. I thought it might be the catalytic converter that was clogged, but I disconnected the exhaust pipe in the front, and it's still misfiring . I don't know if it could be the computer? I bought new spark plugs, and it improved a little, but then it loses power again. It seems like it's an air intake problem or something like that because I have to disconnect the valve on the valve cover to adjust it . Please, if anyone can help me, I would really appreciate it.