02 Rm125 S/n?

On most bikes it is on the steering stem. If you turn the bars all the way and then look at the steering stem from both sides you should see it.

The engine number and frame number should be different. Maybe a trip w/bike to a local dealer could settle it for you.

You have to cross the engine number to the frame number. Although, unless the bike came from the factory race team, there should be a number on the headstock. If there isn't, you may want to have to checked out as soon as possible because when you go to register it in your name, you may have some questions to answer about it. See if it's obvious that the number is taken off or the area is perfect an not touched. If you got a title or MSO, it should have the number on it as well. If you bought it without papers then that's something else. Good luck