1989 Trooper Engine smog and swap

I have a 1989 Trooper that unfortunately cracked the head 3 years ago. A heater hose broke. The temperature sensor never read hot. I noticed when the heater went from hot to ice cold. By then too late. I turned the engine into a water pump. I have it in storage now. I am getting to rebuilding it again. I MISS that car.
Question is, mine is the 4 cyl, 2.4L engine. Just not alot of them around. I saw that most were the 6 cylinder. Can I swap in a 6 cyl ? Would I have to change any of the running gear like trans, etc?
Also, since I live in smogged up the &&^$ Ca, will I have smog issues if I swap engines?
I want to get this car running again. Though it was terrible in the MPG dept, it was a great fun car to drive. Mild off road but fun on the street too.