your backwards on that, The curved fin is what Tecumseh actually called a "low compression" head....its part number 33209. They were used on 25/30/35 in both horizontal and vertical engines, they were made at the same times as the standard head you have pictured with it which is part number 30579/30579a (yours is a post 1976 version though to clear the plug wire). Not something you would find on an engine Tecumseh made for a mini bike.
The only time you see a curved fin one in a mini bike application is the Aspera made ones on the HT's as far as I know (what Broncco was using). They were made in Italy though, those head have what looks to be the same combustion chamber as the standard 30579 heads did.