When the war was over Porsche had re-located to Gmünd, Austria and plans for an own sports car started to take form. Almost all of Porsches work had been done for customers, thus they didn't possess much in terms of intellectual property to base their sports car on.
The whole Berlin-Rome program, including the type 115 engine, was designed with Volkswagen Werk as customer and the Sturmboot program was done for the German military forces. Thus, Porsche did not own these designs.
The lack of own intellectual property was a problem already before the war as Porsche was not allowed to use kdf-designs as a base for any private sports car project. Thus Porsches stillborn attempt of a sports car in 1938 (type 114) had to be designed from scratch.
After the war the situation was a bit different. Volkswagen Werk was now run by the British forces who heavily relied on Porsche knowledge to keep developing the beetle. Also there were some overlap in the staffing. Gustav Vogelsang, the chief engineer for engine development at Volkswagen in the 40's, seems to have worked in parallel also for the Porsche company. According to the book "Porsche - Origin of the species" by Ludwigsen, Vogelsang fled from Germany to join Porsche in Gmünd late 1944, but seems to have re-located to Fallersleben, and his position as head of the engine development at Volkswagen, some years later.
So when it came to re-use Volkswagen parts Porsche had a friendly counter part to negotiate with. But when it came to the re-use of wartime designs I imagine the situation was more tricky. This is only my speculations, but having Ferdinand prisoned in France for war time crime was probably not a good starting point for negotiations to re-use military designs for commercial purposes.
But, as they had the knowledge, the easiest way was probably to do new drawings using new project numbers. It might not be a coincident that the first attempts to develop a new Porsche sports car engine looks more or less like a re-make of the Sturmboot program.
Stay tuned for part 2 and I'll explain more!