help please...engine number?

hi all i bought a fiat punto sporting on sunday,nice car however i took the car to my freinds so he could replace the battery earth lead..upon doing this he decided to have a look at the car..now when i bought the car the previous owner told me that the car had a replacement engine because apparently when the headgaskit went on the old engine they were told by the garage that it would be more cost effective to have the engine replaced. now my mate whos worked on cars all his life and is very good at his job has told me that he couldnt find the engine number on the engine..on closer inspection he noticed a number on the engine,now this number has been scratched of,however you can still see 3 numbers..now i dont know what to do..do i ring the previous owners or do i ring the police to get this engine checked out..the previous owners were very polite people and i dont want to get them into any bother if somethings not right,but if i dont ring them i could be in bother myself. i can provide a picture if anybody needs. (the number is on the engine block at the front just near the exhaust manifold)..