The B20 should bolt up to the auto tranny as long as you use the Flexplate off the B16. You can’t use the B16 intake manifoldm but the exhaust system should work as well as the ECU. The ECU will thow VTEC related codes since the B20 won’t have a knock sensor or VTEC solenoid.
I’d see if I could find a B18a intake manifold. I’m not sure where the newer B18b’s intake manifold mounts a Fast Idle Thermal Valve (FITV), but the B18a mounts it on the intake manifold itself - like the older B16a’s I’ve seen.
If you do decide to get a B18b manifold, check on the FITV
Some of the newer throttle bodies have the FITV attached directly to it.