Bore guide, rod with brush, maybe wrap with 0000 steel wool or a big patch and possibly use some JB Bore Paste and Kroil.
May be a 2 man job but chuck end of rod in drill, one person hold bore guide in place while the other runs the drill guiding the rod back and forth in bore. It may take a few cycles of this and flush with Kroil or other oil to get rid of deposits. Could do by hand but I think a drill would speed the process.
I bet it will polish up, maybe have a few pits or those may lap out, dunno.
