You need to use the proper key. The HB its self really isn't subjected to a lot of strain. It'* only turning the acc belt and SC if you have one. As long as the shaft is not damaged and you use a new key and HB you should be fine. The key is there for a reason, to brake and save the shaft.

Thank you for your help

Ok, I already own a new HB, can I install it after I´ve cleaned it up?
Do I have to watch out for other important things? My first try after I had replaced the old HB would be to start the engine?!

Yep, just start the engine. All should be fine, but watch for any vibration.