A 65–8 taper crank vs flanged crank

[Mark Shayne]

Hello, our Porterfield came out of the factory as an LP 65 (Lycoming engine) During the restoration, it was re-powered with a Continental A65–8 with a tapered crankshaft. The new airworthiness certificate was signed off by an FAA representative with the continental engine installed. (previous airworthiness certificate was lost). I have an opportunity to purchase a relatively low time A65-8F with a flanged crankshaft. In the type certificate data sheet, it doesn’t show the F model as approved for installation on a CP Porterfield.
I’m sure this has been done in the past, and I’m wondering how it was done to keep it legal. Field approval?
Thanks! Mark NC27249