Hi Ian, You might want to just buy the whole a whole Magnum instead of trying to build one. At todays "Magnum" prices, it will end up being more expensive to piece one together than to buy one outright. Just a frame with forks/swingarm on ebay went for $146 recently. And once you get into the build you will relize that there are alot hard to find parts that you will need. Stuff you either have to make your-self or you might get lucky and find someone parting out a magnum. It will take alot more searching and headaches to piece one together from nothing.
I know what your going through though, I searched for a Puch Magnum for a little over a year. And yes, building one is very apealing b/c you don't have wait for one to come up for sale. But if you start building one you will be spending all your money into the project, and while your tied up in that a complete magnum will come up for sale. Pataince pays off when looking for Magnums, after about a year of looking I finally found a Brown Magnum MKII for only $50. Not all Puch Magnums are $700.