Another suggestion is to make it a live axle. Use a ragb to change the drive from horizontal to vertical, use a jackshaft to switch from belt-pulley drive to sprocket-chain drive. So have the belt coming in from the gearbox to the jackshaft, then from the jackshaft to the axle, run a chain drive. Give it a low enough gear to give it torque, but with a 15HP engine, you should get tons of speed and still get a lot of torque.