Trying to do steampunk to the nines (without looking "costumey") can be expensive if you don't have anything at all in that genre! I prefer the grungy steampunk over the sophisticated (like Z suggested), but don't know the feel of the wedding.

If you have a modern kilt, I'd suggest wearing that instead of tartan (the Alt Kilt artificer posted would be great, but any utility style would rock it). Otherwise... one of your "too long" kilts would be a good match for a punkier look.

Clunky boots are your friends. Hrm... pocket watch, goggles... maybe go buy a cheap analogue clock and take it apart for the gears. Hit up a local antique / junk shop and look for anything old and metal

There's a steampunk band called Abney Park... one of the performers wears a tartan kilt. You could do a search on him and model after that.

Good luck!