The engine is certainly not much of a secret, it just uses a very different mindset than the typical racer.
I work for a big construction company as an Engineer/Supervisor, my friends are engineers for Caterpillar, John Deere, Case IH, Polaris, KTM motorcycles, and a corporate jet pilot (for my company).
Our bench racing session had some strange suggestions, eventually resulted in selecting a very common USA mass produced street driven engine from the 40's-early 70's but not one that most people would ever consider for a performance application.