I don't really understand what you're asking. You mean mounting the engine in a different orientation?
Want me to guess? I think there's plenty of other, much larger, forces on the pistons. Mostly, from the rod when it's at some angle to the bore. The weight of the piston is pretty small compared to this other stuff.
You have more to worry about regarding lubrication, how to arrange the oil sump, & making sure the oil drains back down into the sump.