How to tune a musicbox
The tuned teeth (or lamellae) of the steel comb is typically set to a chromatic scale. To change the tuning would require adding or subtracting material to an individual tooth with some trial and error and a good ear. Changing the length would cancel the action of the pins on the revolving cylinder so that is not a good idea.

If you think the scale you have meets your tuning definition then it would be far easier to make a new scroll to change to the melodic and harmonic content you desire. If you are starting from scratch and have the tools you should make a system to be able to change the scrolls so you have a diversity of songs.

Here is a reference to tuning the reeds on a harmonica since this is adding and subtracting material related:

http://www.angelfire.com/music/harmonica/mikesretuning.html

Information on how to tune lamellae as used in a Karimba:

http://www.nscottrobinson.com/mbiratunings.php

Here's a place where you can make your own music box tunes by using 
punchable paper strip musical movement:

http://www.deanorgans.co.uk/order_musicalmovements_mbm30hp.htm

Math part:

http://en.wikipedia.org/wiki/Tuning_fork#Calculation_of_frequency

UPDATE:

I am not entirely certain that using tuning fork math is the right place for a series of metal bars as found in a music box comb so I have done some more searches and found a reference that covers much more here:

http://windworld.com/features/tools-resources/exmis-free-bar-length-calculator/