Why is my car idle not smooth and car feels like it’s choking

So I drive a Subaru Impreza and I changed the engine in it and didn’t replace any parts other then sparks plug when I first put it in even though I should off my mistake which I am paying for now so my issue is the car every now and then after a couple months later feels like it’s choking and doesn’t have enough power when taking off or even when driving my heater core is also blocked so heater doesn’t work unless I unblock it and after unblocking it I realised while waiting for the car to heat up the car while idling feels like it’s about to shut off then the revs come back up and this happens until car is warm enough to idle on its own I also have a pod filter on not sure it that effects anything but ye was wondering what I should be looking for because I have a feeling I might need to pull my engine out again and do the full works on it and there’s also think I burning oil cause my exhaust fumes smell different and very strong as well I am from Australia wa btw