A PPI is intended to provide the buyer with an independent evaluation of the general condition of the car. Many specifics can't be evaluated without invasive procedures which sellers wouldn't allow. Thus, a PPI is, by definition, imperfect but its much better than knowing nothing.
As you note, the condition of the IMSB falls into this category. The only way to know what is really going on is to tear it down and check. Alternatively, the seller needs to provide paperwork proving the replacement. Otherwise, there isn't any way to know the condition for sure.
In regards to cost of ownership, nothing has changed in the 30 years that I have owned old sports cars. Its always been like this...
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