You have no right to interfere in the engagement ceremony of your ex-girlfriend. Your relationship was consensual. You both were in relation with your free consent. It does not confer upon you a right to claim marriage with her if she doesn’t want to marry you.
If you create any chaos you’ll commit an offence and the family of your ex-girlfriend can take appropriate legal action against you. If your ex-girlfriend has cheated you or made false promises of marriage then you can initiate legal action against her.