I don't think anyone has mentioned Enya engines yet. If you're looking for a lightweight, powerful and long lasting 50 size engine the Enya 50SS is my first choice. They are $120 shipped from MRC. Your LHS can probably order one for you as well.

I personally have had bad luck with two Magnum 46XLS engines. This is not a broad condemnation of Magnum engines, just a single data point.