Brad Stevens has a lot of off-season rebuilding to do with the Celts. And so does Bill Chisholm, the private-equity chief who’s slowly rebuilding the ownership team of the Celtics following his successful (for now) $6.1 billion bid to buy the franchise. The Globe’s Shirley Leung reports that a number of wealthy new co-owners are being wooed and brought onboard, including developer Bruce Beal Jr., Granite Communications CEO Rob Hale, and global steel magnate Aditya Mittal, among others. They’re all big Celts fans, apparently. … Current team co-owners have also been offered opportunities to stay. But so far, no mention of Steve Pagliuca remaining on the ownership team, Shirley writes. …