If a 3D printed knob is not as attractive as the original knob, you could cut off the broken stem of the knob and replacing it with an adapter!

Designed to repair a broken knob on a GE clothes washer. ID is 6mm and OD is 10mm.

This model is available to copy and modify in my public folder at onshape:
https://cad.onshape.com/documents/9f5e7d7a0c9ca172a02f6973/w/97340bfa7de4c248675bb071/e/24e21049203c3535be781b5c?renderMode=0&uiState=69acf126836b7aaa107253c0