RE: MPJET Gearbox Broke... Now what????
On a glider, I wouldn't even think of going direct drive, regardless of the other factors involved. Gliders/sailplanes fly much too slowly, nowhere near the 60-70MPH pitch speed of a 6x4 prop direct drive on a Mega 16/15/3. This means the prop is wasting most of its power thrashing about in turbulent air. On something small and sleek it's a different story.
That leaves you with the gearbox choices. Unless the gear lash was way off, another MPJet probably won't hold up any better than this one, which leaves you with seeking out a different gearbox.