My keyhole on the passenger side is broken, it just spins.. So, my question is, can I just buy a new door handle assembly, and have it modified to fit my original key, or do I have to have a new key for a new handle...

 

Hmmm... I think you may have to get a new key for the new cylinder.

I don;t think they can modify a new cylinder to fit an old key. I may be
wrong though.

I think I would try and swap the cylinder from the pass side. I never use
my key in the pass side.
That or get keyless entry...