Whilst not wishing to defend Cessna's use of the O-200 in any way, to correct Rod1 on a couple of points: the O-200 alternator isn't belt driven, it's gear driven off the back of the engine and mags aren't that unreliable and the 500hr inspection is a non-mandatory Service Bulletin.
Why Continental haven't done some advancements like the Liberty IO-240F which is injected and FADEC beats me unless it's purely down to cost.
As for the Dieselair/Howell engine, there is now a very similar engine by Jade/Weslake which was seen at the Wycombe AeroExpo and now at Oshkosh.
If Cessna didn't want to be too radical then go for the Rotax - Americans will soon accept it as a viable powerplant given the use of it in the RV-12, Cirrus/Funk Polaris, Sport Cruiser, Eurostar etc etc.
An O-200...? Barking!