I think that I do understand the point. Actually, I know precisely. This is not my first rodeo. Makes perfect sense to know what one is buying before one buys it. However all too many do not.
If you are looking at the motor then I suspect that you will want to visually inspect it. Also, inspect the documents that prove when and where the 15 HP parts were purchased. Documents that might indicate when the mechanic did the modification. What he did.
In some cases it is easy to verify a modification. In other cases, not so much.
Since Yamaha has offered a 9.9 HP two stroke and a 15 HP two stroke since 1984 through 2009, and you don't even say what year we are talking about, I am unable to proceed further.
If the seller can provide the model identity, or better yet the complete engine ID, then we will know when it was made.