The OHC engine in the roadster is a U20 but both it and the R16 roadster engine are quite closely related to the H20. I think an R16 head might even fit on an H20 bottom end. The H20 is pretty similar to the J series engines and probably evolved from the 1950's Austin engines as the J did only without the siamised ports in the head. They kind of look like a giant A series and I would imagine are plenty tunable as they are pretty sturdy. It's very rare engine here so I'm unlikely to encounter another these days.