the engines we ave today with the water pump on the left sidecover and the starter motor behid the cylinder: came out for the 2000 season. the 501 was already an established engine going back to when the company was founded. they stopped making them after the 2003 model season. the engine was upgraded to koksan ignition and keihin carb. the 400 went to 450, the 470 and 501 disappeared.
for 2004 a few remainr 501's were produced by the oo and SEM ignition. they had upgrade bits but not the tw important items mentioned.
regards
Taffy