Your pic gave me lots of hope, even though it turned out not to work in many ways. Tried an oppy on the mower, no dice, too much hood cutting. Went with a 15.5 Kohler. I did do a similar idler, but its wide, so it pushes on both directions of the belt. I measured with a worn out 1/2 inch belt and got 92.5 inches, turns out a 94inch 5/8 belt is what is needed. Now all I need to do is create a wire harness for power on, start, and stop. Not too worried about aux lights just yet.
I also tried an oppy on the gimphorse, no go w/o lots of hood modifiction there too, so it'll probably get a 15hp Kohler too. Aside from that, tires and an idler pulley, it'll be mostly the same.