I've got a Saito 91 that sometimes for a week or two between runs will have low to no compression. And yet once it runs then shut down has full compression. I probably have a sticky valve but what I'd ask regarding the Magnum is how is the compression right after you shut it down? Nice compression? Still none?

A friend of mine once had a 2 stroke that lost compression between running, which we assume is a sticky ring which I find to be slightly incredible, but evidently possible.

Reading your post again Gray Beard if the valves are OK perhaps, however unlikely, it's the ring.