I managed to get hold of a 2L type 4 engine out of a 914, it had been stood in a yard for a while wrapped in a tarp. On getting the thing to my garage, I tried turning the engine by hand and found the same porblem as you describe. After dissassembly i put it down to rust particles (dust and lots of it) which had fallen from the inside of the steel pipes that fit to the air box (fuel injected).
I now have the heads off and although the B+P's aren't usable I don't think they were responsible.
I take it yours are fuel injected, so could possibly be a simular cause. Also, probably first thing to check would be the fan, housing and assembly! Could also easily cause simular symtoms.
Maybe an idea to remove pipes and check for rust dust from pipes after that. It may sound obvious, but what ever you do, DON'T force it!
My engine (came out of 95,000mile car though) had it's original number (GB**etc), but on inspection of the valve train there was slight but prominant damage to one of the spring retaining collets. Also a pushrod was grooved. There was no other sighn of the damage, so my explanation was that the engine had been rebuilt after drastic faliure (possibly dropping a valve?) There is no end float and the bearings seem solid. Also my oil was crystal to start with but on swilling it around the engine whilst removing tinware, turning engine upside down etc, it got progressivly more dirty!!! All the clean oil indicates is that the oil was chnaged before being stored up.
Hope this helps and good luck!
Dan
