Happily, the Enya two-strokes with a separate front housing will run beautifully in reverse as well. Remove the housing and rotate ninety degrees to the right (when looking from the backplate of the engine):- ie., in the same direction as the standard crankshaft rotation. Your carburettor will now be a little lower, and the engine will look weird, but it's ready to roll.
If you still wish to have "forward" thrust, you'll need a "pusher" propeller.

If you intend to use this revised rotation to provide "pusher" thrust with a normal propeller, make sure the engine has at least one ball-bearing to support the crankshaft, otherwise the engine won't last long; the Enya car motors are great for this type of installation, as they have two ball bearings and a nice big cylinder head heatsink to compensate for the lack of backdraught in a pusher installation.[sm=sunsmiley.gif]