Sorry, the only images I saw were ones with a scooped out section; if it has a full fly, it could be worth the effort.

Well if you can find a matching buckle, it might not be all that hard to rig something up. I assume the adjustable side is on the fly and not the tent... if you can't find a matching buckle, you might be able to just slip a new one on to the existing webbing.

If you can slip on a new buckle (so you'll have two buckles attached to the fly), you could do the rest with just some cord. Get 2 lengths of 12' of cord. Tie very small loops 4.5' from the center, so they're 9' apart, small enough for the end of the tent poles. Tie another loop a bit further out for the stake. With the two cords, you can set up the poles, maybe need to lash them at the top where they cross. Throw on the fly, adjust the poles then stake down.

Now take a 1' length of cord, tie another very small loop, this goes to the end of the pole. Attach the other end of the buckle to the cord. Now you can clip the fly to the end of the pole. You'll be sleeping on the cord, but hopefully that shouldn't matter. With the cord and 4 buckles, it'd probably cost under $15.

If you can't visualize it, PM me and I'll try drawing up some plans.

Good luck!