Did engine swap now get PO455 code, engine misfire and smell

I recently swapped out my 1.8l engine with 187,000 miles on it with an engine from the yard with 55,000. Engine swap went well. Everything was going fine for the first day or so. Then engine light came on. I went to Autozone to pull the codes. I was getting a PO455, emission leak (large). Now every morning when I start the car, it will misfire for about 10 minutes until it comes up to temperature, then it will smooth out and run fine after that. However, there is an awful smell inside the car. It smells like the engine compartment with just a little haze in it. Gas milage is TERRIBLE. I used to get @450 miles on a tank of gas. Went through the first tank after engine swap and only got 360 miles on it. I really hate to take it to the dealer for fixing because the only reason I did an engine swap to begin with was due to the cheap price ($200 for engine and $100 for new clutch kit). Any ideas on how to fix this? Any help will be appreciated!!!