Harley used to build some neat prototypes, mostly in the race shop. In the 1940s they made just one 4-valve overhead valve v-twin that would have replaced the 45 side valve, but they supposedly didn't have the money for production tooling. I saw a picture of it long ago. I wonder if the special one-off bikes will be in the museum, or if they were destroyed.
In the thirties, Harley made some ohv singles in 350cc and 500cc sizes - some were road bikes and some were for speedway. The exposed valve springs and rocker arms collected dirt pretty well. I rode a 350cc speedway version once. In the early 50s (with a different rider) it got some podium finishes against 500cc JAPs. Note: "JAP" was for J.A. Prestwich of England, not Japan.