Simple, just turn it around so the blunt edge is at the front when the engine is rotating clockwise (as viewed from the front of the motor).

By rotating the front housing of the engine through 90 degrees, you effectively convert the intake timing to allow the motor to run in a clockwise direction -- the opposite of normal. This was one of the really cool things about the OS FSR 40 and 45 engines (as well as others). They had a bolt-on front housing so you could convert them to pusher operation really easily. I did it on several occasions with great results.

These days however, most engines have a one-piece crankcase casting so can't be modified without a new crankshaft.

A reverse-rotation camshaft can be ordered for some 4-stroke engines (Saito for example).

If you've got a plain old 1-piece crankcase 2-stroke however, it's just easier to buy a pusher propellor.

As someone else pointed out though, watch for overheating on the ground. The *vast* bulk of the air that is drawn into a propellor comes in from the side -- not the front (believe it or not) and this means that there's very little air "sucked" over the engine when it's on the ground -- so it gets real hot, real quick.