Sam - first the engine needs to be certified, then someone needs to certify the STC for your particular aircraft. Hopefully you have an airframe where enough models have been sold to justify the cost of an STC. If not, you're going to be out of luck.
We've had the same discussions over at the Aerostar forum. There is a huge need and interest for a diesel engine for that airframe. With over 1000 models sold and most of them still in service, it's a viable airframe for a conversion. EPS is a company that are pretty far along in the development of their 350hp diesel. It would be a good fit for the Aerostar. There are others - SMA and Continental's developments.
But ultimately, you're looking at at least a 10 year long process certification rules being what they are. Getting FAA and EASA approval might be one thing, but then developing the STC is another. Remember, it's not just the engine that needs certifying for the STC, it's the prop as well. So the question is - can you afford to wait around long enough for this to potentially happen?
I asked myself that question. Had there been a diesel alternative available for me on the Aerostar today or even tomorrow, I would have probably gone down that route. But life is too short - I want to be able to enjoy the freedom of filling up with Jet A1 today, not in 10 years. To travel and not worry about accessibility etc. Can't wait that long. So I've made the decision to go to a turbine.