It's a bit surprising that the Napier 2-stroke diesel (
https://www.youtube.com/watch?v=D3bj47TAYiU) hasn't had further development. Used succesfully (if a little unreliably) in the "Deltic" railway engine in the UK, its compactness and improved - compared with a 4-stroke - power/weight ratio should make it quite a good candidate for aero purposes.