Cell culture is a little far from my work. maybe @quigly might be able to point you to the name to search for.
A 20x objective would just screw in to replace the 40x that you have. Actually the ‘low cost optics’ version using the Pi camera lens gives very similar performance to a normal 20x objective Comparison of a pi-camera and a 20x optical lens. It will focus through a slide thickness (just)