Obviously they are very simular, but will have a different set up for the oil sump etc, but are the cylinders, pistons valves etc the same between a 27hp horizontal shafts and 27hp vertical shafts?
Yes and no. Per displacement (not horsepower) the parts interchange. BUT if you're intention is to make a horizontal a vertical or vise versa, things get funky. Supposedly it can be done but the effort to do so isn't worth it, and that's what I got out of some Kohler techs. Basically it all revolves around oil flow on the return side.