Hopefully some will come up with the instruction sheet for the kit to give you a copy. It should be pretty obvious if you work from a photo to add the detail.
Finding the Valve gear parts might prove to be a challenge. Other than that, I would replace the detail parts with Precision Scale castings. The AN detail parts tend to be rathter crude.
If you have the side rods ( the ones that connect the 4 drivers) on and the loco rolls smoothly without a glitch you have the major work done. You can then add a motor and get it running.
For boiler - just sand and sand - you might wet sanding to smooth it out. It does not have to be perfect.