This is a completely theoretical, uninformed suggestion, so take it for what
it's (not) worth...

Put the motor in a totally sealed chamber. The motor turns a large disk,
which is positioned adjacent to the rear wall of the chamber. Strong magnets
are placed around the edge of the disk. On the other side of the (thin as
possible) rear wall is a matching disk, with magnets around its rim. This
second disk is hooked to the prop shaft. If the magnets are strong enough,
wouldn't this setup transmit enough torque to spin the prop, while keeping
the motor in a totally sealed chamber?

Remember: theoretical, uninformed! (g)

Alex