I ran into this issue today with an early 2000's tomos. I honest to go dont think it is stamped on the frame anymore, it just gets to foil tags. I know in the 80's it was stamped above the engine on the frame mounts.
Your motor will have a vin however engine vins do not always match the frame serial even from the factory. But sometimes they do. That is located behind your carb understand the frame on a raised section of your engine cases.