Core modifications – do I need to write a plugin?

Hi!
I’m using WordPress blog along with phpBB forum. I made some modifications in WordPress source so that people logged in to my phpBB forum can comment WP blog posts. That works fine, I only had to edit some core WP files (wp-comments-post.php, and so on…).
The only problem is that everytime a new WP version is released and I update it, all my code changes are gone.

What should I do to keep my code changes while always using the newest WP version? Should I write an own WP plugin?