Where is tghe engine serial number or engine vin number?

I had somthing going on with my engine and was told I could either replace the
engine or have it rebuilt. I paid 2950 for a new motor and to have it removed
and replaced. This engine is leaking oil just like the old one and certain nuts
look like they havnt even been touched. I was wondering where they put the
engine vin number so I can have it check by a dealership or somthing.