It might help jog someone's memory if you told us more about the engine? Are the Valve covers silver or gold? Shiny silver or flat silver? What muffler does it have? The cone shaped, straight with prissure nipple on the fron, pressure nipple on the back, or cast? Does it have a sequential letter on it like "A", "D", "G" , etc stamped on top of the right lug?

My 150S has a metal plaque size ID Maybe the tag came off?

Enjoy,

Jim