HI Carrie,

Sorry to hear about your broken key. What a bummer.

Replacing keys can be done, but it's not for the faint of heart. Below is an article about opening up a Tyros 1 and looking inside. Although you don't have an arranger, the principles are the same. Check it out....
http://www.svpworld.com/tyrosinside.htm

Even though I'm very handy, I would take it to my local repair guy to fix.

Hope it works out for you.

Regards,
Al