Obtaining historical records

[ty_wheels]

Hello,

I am buying a 2004 boxster,
the current owner bought the car from a Porsche dealership and was told
that the engine was replaced presumably due to an IMSB failure,
but he has no supporting documentation and the Porsche Dealership involved
claims not to have records going back that far

Carfax also states that at that time the clutch assembly was replaced,
however no information about the engine replacement.

My mission is to know for certain that this car has a replacement engine.
Has anyone got any suggestions on what I could do?