I'll say that I agree with everyone that you should go with a formal dress sporran. I'll also agree that you should get colored hose. I think that black, or having looked at your tartan in a tartan finder, a blue that matches the blue stripes in your tartan. I don't know if you're wearing one or not, but I'll agree that the outfit also screams for a sgian dubh, or at least a sgian don't. I like the fly plaid and ribbons on the shoulders. I personally think that you could lose the belt even without a waistcoat. And the sporran definitely needs to come upwards.

Having said all that, I still think you could go exactly as you are and still look pretty sharp. That's a nice outfit.