Benji got me a 50cc reed valve athena. It's still in transit, though, so I guess I should say he 'is getting' me a 50cc reed valve athena. For me, the point is to build a 50/50 bike, and still have it qualify as a moped in all states. As far as I know, Harold is the only other person who has run that kit in the states. You should email him about it.
As you have probably figured out, a 50cc kit isn't going to be as powerful or fast on a magnum because magnums are heavier, but it would still be cool to see what you could do. See if you can find any info on the 50cc metra on puchforum.net. Babelfish, and a broad understanding of translation issues, as well as a basic understanding of moped dutch will help.
I question the use of a simo on a 50cc kit. I've always felt that the volume of mixture and exhaust being moved by a larger displacement kit was best handled by pipes designed to deal with that sort of volume, like the simonini. However, I'm not sure how the simonini would do with the significantly smaller amount of exhaust being expelled by a 50cc kit. Perhaps it doesn't matter, and the effects of the sound/pressure waves will overpower the effects of the volume difference. I don't know, it's worth thinking about.
If I were you, I'd get the metra, if for no other reason than to provide information to the U.S. moped community on a kit we haven't heard/don't know anything about.