The one I did I didn't need any special tool. I disassembled the freewheel cleaned and greased it on a less rusty donor crankset before installing. There are numerous small bearings involved and thin shim washers to set the bearing preload. With two cranks I had different thickness washers to use for this adjustment. This was a number of years ago so I don't recall all the things required to do this other than i used no special tool.