I see a few options. The first would be to swap the rear axle. The second would be to get an engine that spins the wrong way. Many of the older Detroit Diesel two strokes spun the wrong way. The same is true of many marine application gassers, small and big block Chevy engines being the easiest to sort out parts for. Last, until recently most Honda FWD engines (and many RWD) spun the wrong way, but I don't thing a D15B is going to get a bus moving very quickly.
I'm not sure about transmission choices then. I keep going back and forth in my head. I can't decide if a regular automatic is going to care, but it seems to me you would have some issues with the stator and the front pump working correctly. A manual transmission shouldn't care much aside from thrust forces from the gears being reversed and put on difference bearings.