How should we replace this belt?

Got a head scratcher here. Must have been built around the pulley and belt and now can’t squeeze a belt around the pulley that’s deeper inside as it is too close to the side. Any tricky ideas on how to get a belt installed here? We thought attaching a string or some kind of thread to fish it back thru might work but we don’t see how that would work unless we had a belt that we could take apart and reattach. Clever minds assemble!