Well, you wouldn't be able to convert Mediator's mod to do what you want (i.e. limit the number of topics a guest can read before signing up), because it works on a different principle. His simply limits the number of posts displayed in a topic to guests.
What you want would require either that information about users be stored on the server which tells the server to stop displaying topics and display the message after the user has viewed the limit, or that a cookie be stored on the users computer and the server checks/updates the cookie to keep track of the number of viewed posts and limits accordingly.
The first method takes up server space and isn't really reliable - there are tons of ways to get around it no matter how you do it. Some are things the user wouldn't even have to consciously do - they would just happen by accident (i.e. AOL users might come back with different IPs.) Also, this could cause problems depending on how you do it - for example, if you do it via IP check, it could prevent users of AOL and similar services which share IPs between people from viewing the website even if they've never been there before, simply because someone else has from the same IP.
The second method is useless because the user can just delete their cookies.
In other words, it's a rather silly feature for vBulletin to have, and it's a rather silly feature to bother cloning. The most it will accomplish is annoying visitors.
If you want to restrict guest access so they are enticed to register, Mediator's Gust Read mod would be a far better option, or perhaps make only certian forums guest-accessible, so that guests can get a taste but will be enticed to sign up for the full experience, or don't let guest post, so they'll sign up to participate, or display a few ads only to guests. Any number of other things could be done that would work far better than what you're asking for here.
Edit: Just checked, and vBulletin's mod uses the second method. In other words, anyone who deletes their cookies often, doesn't accept cookies, or knows how to clear their cookies will have no trouble bypassing their mod. It's mostly useless.
