without looking at the voyager.....
there is typically a reason the man. recommends a particular engine orientation--usually because of cowl, fuselage, or landing gear issues. I installed an engine upright on a kit one time (before reading through the instructions) and when I tried to put the muffler on, it wouldn't go on--it kept hitting the fuse side. After reading the instructions, I mounted the engine at a 45 degree angle, and all was well!

and, yes, you will have to make sure the tank height is right regardless of which orientation you choose. If you mount your engine upright, the tank will have to be higher, because the carb will be higher.