Help me convert a dragon!

[Haveatya]

So I would like to use a dragon in my next campaign, but not one of those normal dragons. I was thinking an Outer Dragon from Pathfinder. More specifically a Lunar or Vortex Dragon. Could some of you fine ladies and gentlemen give me ideas on how to convert them over? Are there are rules for conversion somewhere already that work nicely?

stats for both dragons are on d20pfsrd but I am unable to link :L

I'd like them to end up being CR 18-21 anything in there would work out fine! Thanks for any advice you have.

[JackPhoenix]

You can modify existing dragon...just exchange its abilities to some better suited to what you need, or if you're especially lazy, just use dragon from MM as it is and refluff the effects of its abilities.

There's Kryx's http://marklenser.com/5econverter/, but even that still needs some manual tweaking