by changing a 2.8 out for a 3.1 litre in a 19890 chevy cavalier, do u need to change any mounts or anything serious????

This is a fun lil project. I did it my self last summer. Nothing needs to change really. They use the same computer, well kinda. If you have the old school vacuum EGR on your 2.8, u need to put that on your 1.3 and also change your Throttlebody so that it can control the EGR. The reason you need to do this is because the computer in your 89 doesn't know how to control the electric EGR. The throttlebody off your 2.8 has a lil hole in it so that it can use the vaccum from the inner throttle body to control that EGR. The 3.1 engines don't have that. I found that out the hard way.

If you want to i recommend taking the caps off both of your throttelbodys and checking them out and clean them. Its important for them to be clean. Any blockages can make an easy emissions fail. Get new gaskets for everything you change too. A vaccum leak is real bad.

You may also have to change that wiring harness for your fuel injectors. I had to change mine. I got my 3.1 out of a 1992. Mounts should be the same.