As suspected

As I assumed in the very same moment Gary ( the guy who forced me to buy it from him) told me it had a knock in the engine, it's got a worn crankshaft shell. Meaning I'll have to crack open the engine and replace it.

I got the sump out today and managed to reach the  con-rods big ends only to verify that there's play in the left one. Hopefuly there will be no damage to the crankshaft itself , it will be just a change of shells.