Recently we have discovered an awesome tool that was hidden at japanese.simutrans.com links section.
Its name is Resizeobj and it allows you to port almost every kind of object from pak128 scale to pak64 scale, with very good results. Take a look..
It’s a command line tool for Windows with several parameters that allow you to set the amount of anti-aliasing, special night colors and the image size among other things. You can convert vehicles too. The windows binary and the source code are available for downloading at resizeobj homepage.
P.S: For non-japanese speakers, I recommend to use an online translator as the whole website is in japanese.