To finish up a point from another thread...
If you are going to replace an engine case with an engine case with no serial number ( a blank case), you can stamp it with the original engine serial number. But, you should prepend "AT" to the engine serial number.
See this thread:
Porsche356registry.org - Engine S/n
To quote a poster:
"AT probably means "Austausch" ( German for replacement )"
This is the only ethical thing to do. Especially on a car where having an original engine would affect the value of the car.
Scott