Drill holes, add raw bolts. Attach timber supports and re-site current roof.
I'd also glue rock wool to the underside of the tin roof after installing a frame of 2x2 stud work.
Then plasterboard over the insulation and install some lights.
I'd go for 4x2 timber extensions each side of the current columns.
So 4 raw bolts for each column, 2 either side.