I have flown many pleasant hours behind Lycoming and Continental engines as well as with Pratt&Whitney and Wright radial engines, but their time is drawing to a close. If you wish to fly in the Artic regions, you may have great difficulty in finding AVgas at any price. You may find that the green fiends have made it impossible to add any lead content to your Avgas, and that your engine actually needs just a touch to keep running.
There are new diesel engines on the horizon that are capable of using a wide range of turbine/diesel fuels. My personal favorite is the DeltaHawk engine which has been under development for the last 20 years.
It has been a painful process to watch from the outside and has cost a small fortune, but they are drawing very close to the certification target. It is not enough to have a brilliant concept. In the end, you need to develop a wide base of engineering knowledge, qualified suppliers, proper industrial methods, proper trust from the certifying authority, etc. A very daunting gauntlet for a start up company. I wish them well.
DeltaHawk DH180A4 | DeltaHawk Engines