It's closer than we think.
Who is "we"? It might be closer than I think, but it is certainly not as close as you think!
For experimental designs it sounds interesting. Give the EAA members and their likes a couple decades to sort it out, and then we can start discussing it.
For normal category aircraft: No way this side of year 2035. We haven't even got the diesels sorted yet, despite their obvious theoretical advantages, and that technology has been around for a long time.
The development costs, including getting it certified, would be astronomical considering the tiny volumes of light GA today. The days when any significant technological advance could be paid for by users of light GA engines ended when taxi, business and light airline operators went turbine. What we can hope for today is reusing some parts of what the automotive industry comes up with, but since their operating environment and requirements are entirely different, most of their technology can not produce a light GA engine that is better than an IO-360.