Methylene Chloride is great at this. However, is is highly toxic and is a carcinogen (cancer causing) chemical. If you do use it, please wear the proper gloves and do not get it on your skin.
NASA banned it from their facility, however, I got an exemption for the carpenters to use it in the shop. It was the best glue to make plexi display cases. There were only two people who we allowed to use it, and the supply was kept under lock and key.
I would not try and spread the crack apart. This could cause it to run even more then it has already.