I would be EXTREMELY surprised if you couldn't ride Magnum. As long as you pick the front row of any car you shouldn't have a problem since there is a lot more leg room. Of the over 2 million people that rode Magnum last year I witnessed only 3 or 4 people all season who couldn't ride because they couldn't get their seatbelt buckled. You wouldn't believe the number of people who came to our ride and commented on the much longer seatbelt length in comparison to MF and Dragster. The good thing about this (for our ride numbers) is that these people would often come back and ride multiple times since the wait was generally short.
If you decide to go to Cedar Point, I doubt that you could ride any of the new Intamins (Dragster, MF, WT) or the wooden coasters. The woodies' belts are way too short considering they have that retarded seat divider in the trains.
It's still me, here from the beginning back in 1999. Add 1500+ posts to the number I have in the info section if you care about such things.