I replaced engine, now I have lean code, AC compresser will not turn on.

As far as the ac goes when the ac hose was not disconnected and moved around it could have been brittle from age and caused it to leak.I would do a pressure test on the ac to see if the charged leak out.

Now for your vent issue and lean code I think there related if they happened at the same time?For the lean issue let the car idle get some carb spray and spray around all the vacuum sources vacuum hoses intake gaskets etc. If the rpms go up down or the engine stumbles or dies you found your vacuum leak.

So the ac worked before the engine was replaced as well as the ac vents?Also was there a lean code before the engine swap?So you checked the ac pressure to make sure there was enough pressure in the system to allow the ac compressor to turn on?Also how did ou check for a vacuum on the hoses and what about the intake gasket etc?

Not answering all my questions and not being able to see the car makes it really hard to help you.I will give a back ground on myself. Iam a factory trained saturn tech a gm EV 1 full electric car factory trained tech I was a team leader and shop foreman for saturn ran the used car department for the service end of the used car department. Iam not just a diyer on here answering questions I go out of my way to help people on this site. If you dont want to work with me and answer my questions etc then I really cant help you.