Why am I getting exhaust fumes in cabin

After searching the forums I can’t find anything that sounds like my problem.

I have a 2013 Prius 3 with a 2016 engine. I replaced the bad engine with a 2016 engine with 55k miles. I’m starting to think I have a cracked exhaust manifold or something. Every time I turn on the auto setting for the ac and it uses the outside air I get a massive amount of exhaust fumes while idling somewhere. It has started more now that it’s cold and I’m using the heater. I replaced the manifold gasket and even sprayed it with liquid gasket maker to get it extra tight. I ever replaced the donut gasket and the spring bolts hoping that would solve the problem.

After reading through the forums it seems like a lot of people have issues with donut gaskets but it hasn’t helped me at all. I’ve looked around the manifold to the cylinder head and see no signs of leaking but it’s starting to make the car throw the p0300, p0301, and p0304 code. I was assuming the misfire came from the leak but I’m thinking the next step would be replacing the gasket at the manifold. It’s driving me crazy bc it just seems like one thing after another. It went months with no issue until it got cold and I started smelling all those fumes.

please help. Any suggestions are welcomed.