You did not say what specific Mauser you had or what specific Mauser the replacement ejector came from.
Mausers look so much alike that we often take interchangablity of parts for granted, but this is not always so.
I once lost a Mauser ejector. It went flying off into some part of my messy basement when I tried to disassemble it and, try as I might, I could not find the little devil until it popped up months later when I was doing something else.
I got a new ejector from a man that sells parts at local gun shows. He had a whole bunch of them that varied in just about every conceivable dimension, including the slot holes. I bought two that were rather different and the first one I tried worked O.K., but I can see were this might make a big diffence if the ejector were to one side of the tolerance scale and the rifle at the other.
Admittedly, the odds would be against both the ejector in the rifle and a replacement being too small in some critical dimension, but it is a possiblity.
You might try taking these ejectors to a gun show were parts are offered or to a gunsmith who has a little stash of these, compare them to what you have and buy one or two that seem to be radically (by comparison) different. This might be easier and cheaper than trying to modify the ejector by welding, unless you are good at such things.