Migration from Postgreysql to SQLite

I am working on a project which uses PostgreySql on backend to handle database. As PostgreySql have Procedural Languare i.e. PL support, existing code contains various User Defined Functions.

Now I want to migrate to SQLite which does not have PL support, I think. How can I handle this situation in SQLite and use same code fragment (i.e. functions) different time by just calling it.