Store signs converted to billboards?

Hi! I am seriously considering starting a billboard company in eastern PA. I am noticing a fair number of abandoned store signs from companies that no longer exist.

Can I convert the store sign into a billboard? Specifically, can I remove the store sign and use the pole(s) as the mounting structure for a billboard (of course it would be right sized based on the load the pole can handle) or will the pole have to be removed and replaced with a billboard specific mounting structure (i.e. monopole/wood or I beam frame)?

Thank you.