Changing the tumbler has been discuss many times, the process can be found by doing a search. The process only works if the tumbler will turn, if the key is broke off inside and you cannot turn the tumbler then the tumbler will have to be drilled out.
How much of the old key is in the tumbler? Might there be enough that you could manipulate the switch and get it to turn? If so you might then be able to use the procedure you find during your serach. You can get the proper ignition tumbler and keys through the dealer, all you need is the VIN and proof of ownership - these keys will be the same as the ones when the car was built.
Hope you can get it to turn.
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.