I've never seen any after market parts for this kit. I do know the shuttle included in the kit is the same as the separate 1/444 scale model you can buy.
I bought and built this kit when it first came out. The most frustraing/confusing part is building the tower, many of the parts look the same. Very easy to glue the wrong parts together. Its not that it is difficult but you just have to check and double check that you are assembling the correct parts in the correct orientation, very easy to get a part upside down. Probably the hardest part (and is not THAT hard) is making sure all the subassemblies are square and true or the tower will be all cockkeyed and look like a hurricane twisted it.
I seem to remember the fanatics bah humbugging the accuracy of the kit but to heck with them. Those types really get on my nerves! Where are you gonna find something more acurate than this unless you scratch build and very few of us have the time or patience for that. Just build it and realize 99.99999999999...% of the people in the world will not question its accuracy
I'll try to find it but there is a diorama out there someone built that includes not only the parts in that kit but also the surrounding cement bunker and road leading up to it. Its very nice and BIG!