An engine includes an engine case split into an upper case and a lower
case. A crankshaft and a transmission input shaft are rotatably supported
at a mating surface between the upper and lower cases, and a transmission
output shaft rotated by the transmission input shaft is supported in the
lower case. One side face of the lower case has a circular opening
through which the transmission output shaft can be inserted from the
outside. An eccentric bearing holder holding a bearing that rotatably
supports one end of the transmission output shaft is mounted in the
opening in a detachable manner from the outside. The axis of the
transmission output shaft deviates from the center of the eccentric
bearing holder toward the transmission input shaft. The inner diameter of
the opening is larger than the outer diameter of the largest one of
transmission driven gears of the transmission output shaft.