I just went through this on a 2004 Bonneville project bike. There is no way to know what key your lock needs unless you have the key code that came with the bike. If that and the key is lost, you will need to replace the lock and key. I purchased replacement lock from my local Triumph dealer and installed it on my project bike here: https://triumphbonneville.org/project/2004-bonneville/
Really getting to the point I need to do this to my old America. Have to keep everything locked up around here anymore. That lock has not worked since before I bought the bike 6 1/2 years ago. But the neighborhood is changing. ...J.D.