should I take the magneto off over winter or leave it wrapped?

I'd double bag it and call it good. I think that you'll be fine leaving it on.

Thanks. That's a good idea. I would like to leave it on because its not so easy to get back on and if the impulse drive gets moved then I have to retime everything.

I wouldn't bag it. Any moisture that might be in there will stay in the gag. I would make some type of cover that would let air flow but keep any water or snow off of the mag. Just my 2 cents.
ChuckC

Chuck makes a good point, you could cover the top and leave the bottom open the drain/breathe.

I was thinking about the moisture also. I will cut an opening in the bottom of the bag to allow air to circulate but not come in from the top

A lot of farmers here used to cover a distributor with a rubber glove, a hole in each finger for the wires and where you'd stick your hand in left open and loose around the base of the distributor. The fingers were punched with a small hole to stick tight to the wire. It pretty much eliminated moisture problems from water, yet the cap and wires could dry out. Something similar should work on a magneto if it was big enough to cover. We also found that synthetic rubber inner tube carries electricity, so not quite so good.