In a brief look around I couldn't find any numbers like that. "Older motor" doesn't help much. Horse power would help.
I'm 70 so to me a older motor would be 40s or 50s. A picture sure would help. A guess for the decade might help.
An 18 year old a 1980s would seem older
18902 comes up as 1968 18 hp.get the numbers rechecked to make sure and even pics would help a lot.ask the seller the hp. does 18 hp sound right?you have too many numbers in the first ,it was the closest I could find.