This is something I've been playing around with for a few years. I sold my earlier version of these two figures a year ago, and I'm excited to have some new pieces to work with from recent Lego offerings.
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Ripley still needs her grenade webbing and weapons (I've got to order one of those flame throwers!), and Citizen Brick makes custom Lego babies that would probably offer a good option for Casey's head. Their heads are made using the same size piece as the amber light on the top of the Power Loader.
The real improvements are Newt's hair, Ripley's face, and actual short sleeves. That last one has been on a lot of folks' wish lists for years now. Before you had to either paint the arms or try to glue or tape paper onto them. Neither was particularly convincing. Now thanks to the Simpsons line we have a couple different short sleeve options!