I don't know what the tartan is, and all I'm getting at work are the "red Xs", so I won't even hazard a guess.
Having made a kilt with a "regimental strap" (Hunting Stewart in the Gallery), my recommendation is leave the buckle as it is, replace the belt/strap with one that is long enough, and remove the hip strap and buckle. For the new strap, contact Cavscout at http://www.oconeeleatherworks.com/ - he should be able to whip something up for you if you don't want to make the belt yourself.