Flathead Briggs once in about 1986 before I hit a tree. Flathead Briggs for a year when started again in 1992.
Lots of different engines from 1993 until 2001, when I stopped racing.
Then 4-strokes when I got back in 2011-2012, before going Rotax in 2013, and shifting gears with a TM K9b this year. If I didn’t already have the Rotax and the shifter, I’d be running a LO206 this year as well.