After recently completing my little bruder bulldozer conversion I decided to next have a try at making an excavator.  I began with a Bruder 320 and began cutting plastic.  I will post pictures below as I make progress.  I am teaching myself how to 3d model using Fusion 360 (free) and then printing the parts on my 3d printer.  I will be using modified servo motors for the tracks as well as for slewing.  I ordered a magom hydraulic pump and I am going to attempt to make my own cylinders and valves when I get that far. 
Mount for track motor.  

Hub Adapter

All Glued together