I don't have a portfolio. I have found a job listing that is looking for someone with skills that fit what I do to a 'T'. They are asking for skills with an engine that I mod for. Would it be appropriate to send them those assets (with a cover letter of course)?

Yes I am franticly puting together a portfolio.

You will have to have a portfolio to get an art job. Just do some renders or screenshots of any models that you can show. Show textures and animations if appropriate. I wouldn't send them the actual model files unless they specifically request it, and/or you and your mod team are totally cool with giving the models away.

If you set your mind to it, you can do some renders, gather some textures, and put together a portfolio in a few hours. If you have any concept drawing or fine art, you might want to throw a couple of those pieces in as well. Don't go overboard, just show your best stuff. If that happens to only be 7 or 8 pieces, then so be it.

You can also use a program like Fraps to do a video capture of actual gameplay footage to use in a demo reel. That way they can see your models from all angles and with in-game lighting.