I have a 1960s trilevel home with a bathroom in the bottom level. I am currently updating the bathroom and would love to remove the drop ceiling and return to the original drywall ceiling that was present when the house was built. However, in the last 5 years a fan/light unit was installed in the drop ceiling. I am wondering if it is possible to install the bath fan into the old ceiling and if so how much clearance is required between the light/fan and the upper floor joists. Would this clearance be covered under my local building code? I know this is outside of my skill set but I am not sure who I should call to make this change. Suggestions?

Thanks for any help.