I don't think the crank height difference is enough to engineer for. If the engine mount holes are the same, only needing enlarged, I would use a drill with a pilot OD spun on the tip. I think you can change the shim pack on the trans clutch to adjust the belt if needed.
I don't think I've seen an engine where the crank wasn't parallel to the block mountings.
Great thread by the way, a labor of love, Sure makes me appreciate the simplicity and spaciousness of an air cooled briggs vanguard. But the big water cooled kawasaki will be torquey and quiet with power to spare.
To Invent, you need a good imagination and a pile of junk. (Thomas Edison)