Find a suitable size box that has enough space around it so the engine does not touch any side of the box. Take three pieces of card board that are the exact size of the inside bottom of the box. Put one piece down first inside the box to help strengthen the bottom of the box to support the engine weight. Stack the other two pieces on top of each other and reverse your engine mount bolts to mount the card board to the bottom of the engine. Tighten the bolts down enough to secure the cardboard but not to rip through it. Slide the engine in to the box so it is secure. There should be no slop and engine should be centrally located in the box. Take off any part that can get broken off (plastic tank, air filter, spark plug, etc.) wrap them up or place them in a zip lock bag (zip lock bags help a lil in masking a gas smell from the gas tank), cover any holes with masking tape (spark plug hole, carb air intake, exhaust hole, etc.). Use bubble paper or packing paper to pack in and around the engine and parts removed for shipping. Do not use foam peanuts the debris from the peanuts can get in the engine. Use the packing tape that has the fibers runnng through it, its stronger than the regular clear packing tape and will help secure and strengthen the box to accomodate the engines weight.
hope this helps