Not sure about this model but if you look on the engine block there maybe an aluminium disc or core plug ,with omc engines johnson/ evinrude the engine numbers stamped on that .
No they dont have them on the smaller outboards M chappelow but I did have the number 8 on the block of the 8HP where the 6Hp had nothing and without a vin plate it could be very hard to tell them apart it could be a 6HP and just have a 8HP cowel, I found this site a good read :www.leeroysramblings.com
How comfortable are you on buying a motor without its serial number ? On the newer motors it's sometimes understandable, as they use stickers which could get power washed off etc but on that age it might have been a metal plate riveted on. Be sure to do your checks. www.stolenboats.org.uk is a good starting place