Along the lines of what these guys are suggesting, stick it together with JB Weld, screw some straps under across the cracks with countersunk screws from the top, leave them long, then invert and fill with concrete. The screws will help bond the concrete in, though it would probably be sufficient without. It'll be heavy but, that's a plus other than for moving it. A cross of 1/2" rebar, corner to corner set in the concrete wouldn't hurt either. Fill any bad spots from above, sand and paint.