I plan on doing the same thing to my Tweetsie 12. I did some look into it and basically the chassis on the standard locomotive is the same as the one with metal valve gear with the only difference being it has metal side rods. If not you could always get the chassis and pull the drivers/side rods off and install them onto the old drivers. Hope that helps! ![Smiley :)](https://www.bachmanntrains.com/home-usa/board/Smileys/default/smiley.gif)
![Smiley :)](https://www.bachmanntrains.com/home-usa/board/Smileys/default/smiley.gif)