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You could purchase the Universal Binary disc image for $5.00. I don't know if the old PPC version can use that same disc image to get the game to load, but there may be no harm in having a Universal Binary version of the game. I think the latest update has the UB PPC performance up to par.

http://macgamestore.com/detail.php?ProductID=616

That, or you can try to contact MacSoft for a new disc. They probably have some kind of disc replacement program.
 
There's really not much point trying to play the computer version of Halo with a gamepad anyway, since they took out auto-aim and bumped up the difficulty. Just use mouse+keyboard; it's more precise.

--Eric
 
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