Custom-built by IAME, "in the style of the TATRA". Some have called it a TATRA engine, but it may just be a close copy. They may have only made 2 of them. The one in the convertible was found in a pile of scrap and restored, if I read correctly.
I first saw the engine in a video of the convertible running, and thought "what kind of supercharger is that?". Further reading disclosed it is an air-cooled engine.