I have seen 914s with Chevy small block engines. I have never seen one that was well done, always a hack job.
Converting a 914 to a 914-6 is not real hard except finding the sheet metal for the engine. Still not cheap. One of my friends had a 914-6 that he converted to a 2.7 liter engine. That was a new engine back in the late 1970s.
Of course the coolest engine conversion on a 914 was done by the Porsche factory. They dropped a race car 910 - 8 cylinder engine in two of them. They welded a steel roof on it to keep it from flexing too much.
https://www.conceptcarz.com/z24885/Porsche-914/8.aspx