What needs to be done depends largely on how much it needs to be taken in. If we are only talking about an inch or two, then this can easily be accomplished by moving the straps and buckles a bit. You don't have to be a kilt maker to do this, you just have to know how to sew.

If it needs to be taken in by, say, more than three inches, it would be best to send the kilt to a kilt maker willing to do alterations, with a new set of measurements. They will probably charge by the hour, and how much it will be would depend on just how much re-tailoring would need to be done.

If you don't know anyone in your area, let me know and I'll get you some phone numbers of people here in NC I know who do alterations.
Aye,
Matt