I had a kilt a long time ago that I had to resize. It had 15 knife pleats in it, and they were each two inch deep pleats. All I did was take a seam ripper and remove one of the pleats to give one of the aprons some extra length. Removing buckles and resewing them on should also be able to give you a little bit of reach as well. I'm sure with the combination of the two you should be able to get the kilt to fit.

If you posted a few pictures of the kilt "wearing it, a pleat shot, and a front shot, and a shot where the buckles currently are" we will be able to help more with it.

BTW