Scale Trains Conversion Kit
If you are looking to convert ScaleTrains electronics to SoundTraxx electronics, a Decoder Buddy V5B with 1.0K resistors is a common way to convert to a 21-pin SoundTraxx decoder. A Power Bridge III with two 1-Farad capacitors is also a good way to replace the stay alive. A Scale Sound Systems speaker is a popular and common conversion. This is a kit that should assemble all the parts you need to make the conversion.
Here are some videos that address some details of the procedure: