Obviously you guys haven't been married very long.
There is an art to this:
1. You simply don't get in the situation where you are asked. (Hide the Kit, motor or whatever. Bring it out after she's been on a walmart spree.
2. Have the box delivered to the office.
3. Drop hints well before a birthday.
4. Show her the plane, tell her that you have had it awhile, that you are saving for a new motor. Begging for sympathy works wonders.
Use your imagination, she does.