I first tried to replace the key.
As I live in Belgium they have no stock of imperial keys only metric, so
I bought a 5 x 5mm oversized and filed until it fitted on the crankshaft.
Stange,it did not fit on the flywheel, so I filed half of the key to fit the flywheel.
Strange that it is not a square size.
The key fits now in both, but is asymmetric.
The engine doesn’t make a knocking noise anymore on a first test.
As it rains today I hope to cut the grass tomorrow, let’s hope it stays ok.
I will let you know in a few days if everything is working.
Thank you very much for your help.