Hallo Anders,
I am trying to do exactly the same thing as you - update my old 171 train with the RC motor, and would also like to first try it out with an old 9v battery box before buying the new, rather expensive rechargeable battery. There are two things I'm not sure about. Perhaps someone here could help:
1. Which 9v old battery box can i use, the 4 x 8 stud one (http://www.peeron.com/inv/sets/5391-1) or the bigger 4 x 14 one (http://www.peeron.com/inv/sets/5293-1)?
2. I would prefer to use a shorter cable than the current pf one, for instance the 8.5 cm one (http://www.peeron.com/inv/sets/5041-1). Would it work with the rc motor?
Good luck with your project Anders!