I see two approaches that come together in a way that suggests that a diesel rod that is at least traditional in spirit is possible. The one is the relatively new field of diesel performance, especially where turbocharging is involved. Diesels were after all turbocharged long before SI engines were. There are a lot of middleweight commercial engines nowadays that have a lot of performance potential. The other is historical diesel engines, of which many have some sort of uniqueness of character or quirky charm. I'm thinking of two-stroke Detroits, Commer TS3s, Gardner 8XLBs, air-cooled Deutzes, horizontal underfloor bus engines, all manner of obscure stationary industrial or marine engines, etc. etc. etc. Many of these would have limited performance potential, though being as a rule overdesigned might have more than one might think. For one thing diesel tuning is about torque rather than rpm, so long strokes are quite viable. My personal holy grail here would be one of the few "mini-Jumo" opposed-piston bus engines Junkers made in the '30s.