The locomotive tpe is not mentioned unless I missed it. For a steamer I agree that if a replacement in 2-rail is available that is a good way to go.
If the loco is a favorite diesel model that is not readily available or whatever, another approach is a complete new drive train. P&D hobbies still sells complete drive-trains and parts based on the old Weaver single motor drive design. P&D has had various parts remade. On some locos this is almost a drop in. On others you have to do open some space on the frame for a gear tower. On many you have to make a truck mount bolster and locate it and attach it to the frame. But these are relatively easy jobs with typical modeling handtools or light power tools.
