I've never even heard of a mini Magnum.. i designed something like one... but ummm if you mail Alan theres a slim chance he might have something kicking around, but personaly i'd just shrink one down by 20% ( at the most ) and just put pen to paper.
I dont think you'd be disapointed if you put a West 36 in one tho.. so much lighter its not gonna be slow.
BalsaCabin sell the Demon which was the pre-runner to the Magnum.. if you got a plan for that a scaled down version is only a button press away.
http://www.zyworld.com/balsacabin/Page3.htm