I have used plywood placed on the ground to pull an engine on my tractor. Worked just fine. I even had to move an engine on a stand across my yard by using two pieces of plywood and leap frogging them. Seemed very safe to me. It just takes a long time moving less than eight feet at a time. Of course there were no hills to deal with. That would have been a disaster.

A day without laughter is a day wasted- Charlie Chaplin
When wrestling a grizzly bear, you have to keep at it until the bear gets tired, not when you get tired.
1948 Chevy 2-Ton