Even so, I used to need to remagnetise motors about once a year, especially if I was attending exhibitions with the models. I doubt any stroking with other magnets would help in the long term.
My memories of the Triang machine are of a *large* coil, which must have been several dozen Henrys in value on a horseshoe shaped soft iron clamp that closed onto the magnet end of the motor, while turning on a DC supply to the coil. The visible part of the whole thing was probably about the size of a food mixer. The field was strong enough to reverse the magnet's field if it was accidentally inserted upside down, with embarassing results next time the loco was used. The operator had to keep his watch well away even after it was turned off, too.