Newbie - advice re repairs to old Thomas Trackmaster trains

it also depends what batteries you use. some are more prone to corrosions. tender engines are the hardest to repair given you have solder the wire from the tender to the motor to the front (the fire box )

depends on the character, some are not worth repair (i.e. thomas, percy)

given I am not an expert in extensive repairs, i would harvest the motor/battery compartment off duplicates. some engines have the same design you can swap the not working ones with working ones