RE: Why don't we have more 4 stroke gassers?
More weight and less power are not always such a bad thing. Especially for a WW1 scale model. The ability to spin a bigger prop, with great fuel economy, with the really cool sound, would be a nice thing to have. I think it would be cool to not run pre mix fuel as well. There has been much talk in the past about those honda motors, but unless you convert one to aircraft use yourself, there is nothing available.
The Roto engines are out there but are quite pricey.