Engine cradle

Not sure how robust you need this to be but depending on how much time/$ you wanted to invest, you could make one pretty easily.  I've always used an engine stand to roll my engine around but I could see an advantage, long-term, to being able to push a storage engine around on the floor when needed. 

 

Your engine should have a flat oil pan with a skid plate on it.  A stiff piece of plywood would hold the weight.  1/2" thick may be enough but 3/4" would definitely be.  You could attach wheels directly to the plywood or buy a dolly, e.g. harbor freight has one that is rated for 1000lbs (sale this weekend).

 

 

Then  lay the plywood on top.  You could even measure the width and length of your engine and match the plywood so that it doesn't take up more floor space than needed. A cargo strap around it will help eliminate tip overs.