The Black Fin engines were different from most modern engines.
Today's engines have a cast aluminum case and a smooth, slip-in sleeve.
The Black Fin engines had a cylinder with the fins machined in place. These engines in my memory, include the OS Max (up to III), K&B Greenhead and McCoy Redhead. Johnsons were also built this way, but they were so high-performance that anybody with a healthy one would blow out the field.
There were also Veco engines of that era, but I don't recall if they were black fin or sleeved engines, but they will be allowed.
Around here, the ST C35 came in 1962 with an aluminum case and a slip-in sleeve, beat everything else, and we never bought a black fin engine again.
There are plenty of Fox 35 Stunts available, so nobody should feel excluded for the lack of a 55-year relic.