As long as the break is clean, and it fits back together nice and smoothly (no missing bits or bent plastic) I honestly have had good luck to fix a key or two simply with Superglue. The trick is that it has to be a VERY good fit, and also give the glue plenty of time to cure well (it says you don't need to, but trust me, it does!), say a day or so, before you play it.

No guarantees, but give it a whirl. The sensors for velocity are independent of the key itself, so if the key fits back in place with no noticeable difference from the other keys, you should be good to go...

Just remember, Superglue needs a VERY clean surface to work it's best. If you touched the surfaces that join together, they will need some cleaning with a little denatured alcohol (on a Q-Tip) to get any finger grease off of there.

Best of luck...