One of my inner fendwells is plastic. Im going to be painting the engine compartment and was wondering if i could paint the fenderwell with the same paint as everything else.

I had plastic inner fenders and cleaned them off and sprayed them with plastic primer and shot them with chassis black out of a spray can.
Got it from NAPA. They didn't look too bad, but I eventually replaced them with the steel inners.
Look much better.

Just an FYI, if you are going for originality, they should not be painted.