[Toolman]

Some options when you have two sized keys on the same shaft.
option 1- broach the existing pulley/gear to 1/2" if you have a broach.
option 2- order what is called a step key which will have two sizes in one key. (MMC) can usually get it to you within 48 hours or less.
option3- get a 1/2" key and machine the other side to the size you need thus making your own step key.
PS: Are you sure it is a 1/2 inch key this is a very big key for a large shaft usually for much bigger shafting ?