Me, I'd contact the manufacturer and try and order a new keel to replace the one that has been ....torn, stretched, twisted and generally no longer able to do its job. If riveted back it will simply twist the hull creating many stress points. To remove it and to try straightening it would be allot of work and even afterwards, the holes to re-attach won't line up IMO. Haven't checked but perhaps aftermarket replacement keels are available online.