I have never bought a Magnum, I have worked on a lot of them for others. Early last year was the last one I had in my hands. I keep my engines for decades and just rebuild them when needed. When I get a Magnum in trade I give them away, I won't bother rebuilding one for myself. I have stuck with OS and YS for my own use. My first choice is always YS. I just mailed off an OS 120 pumper to a friend because I have several YS 120s on hand and will never use it again. I have kept my OS 91s though, not for sale or trade. Last OS I sold was the 70 and I have been kicking myself for doing that for years. If I could I would go 100% gas now days but you can't make any of the money back from a used glow engine, even the YS or OS.
If you want to get a good engine buy a used OS and rebuild it for your own use. I find YS cheaper to rebuild but they aren't for everyone either. Last new engine I bought myself was a 91 SK two stroke. I find them much better and more suited to my use then even the OS two strokes. For $100.00 a pop and with a muffler they are hard to beat. I have only had one bad one, the little end of the crank broke on it. Only took two weeks to get it back from service and it has run perfectly ever sense, not one dead stick but none of my engines ever dead stick on me.
Magnums were the rage in my area a few years back and I had a lot of them on my bench. The carb design is an old one and not very good. I just really think they are very poorly made.