Hmm, I don't know enough to give you a confident answer yet. The provided knowledge sources do not contain information about replacing Kratos with a custom solution developed using Ory fosite. However, Ory Kratos is designed to be customizable and flexible, allowing you to implement your own user management, login, and registration in a secure and straightforward way. You can also implement your own custom flows if you have specific requirements for your users' experience. For more information, you may want to check the [Ory documentation](https://www.ory.sh/docs/ecosystem/projects#ory-kratos).