Well,
I think I am onto something. I noticed a hissing sound coming from the top of the engine when it was being turned slowly. It turns out that the valve cover has a small crack near one of the screw holes, and air escapes off it.
As it happens, the seller mentioned a crack that had been taken care of. In fact, it has been epoxyed (or JB welded), but clearly it did not stand the conditions during run.
Fortunately, a new cover should not be too expensive. Since I am after parts, anything else I should order just in case? I was thinking about the regulator spring, a new diaphragm and a gasket set...
Also, I read somewhere to slightly roughen the bottom of the valve cover to ensure a good fit with the gasket. Is this a good suggestion.

One more thing. Is it normal that, without glow plug and turning the prop slowly by hand the regulator opens only every other revolution of the prop? My understanding is
that the pressure to open the regulator is controlled by a small opening on the crankshaft, hence my confusion...

Hopefully, I will be able to get the cover in a few days, and I'll get back to you with some good news.
I thank everybody who contributed. I learned a lot...

Alberto