When a synchronous alternator loses residual magnetism, customers complain about having to flash the fields. First, check the exciter fields to see if there is any residual magnetism. If there is, swap the field leads at the AVR (automatic voltage regulator) so that the AVR does not cancel the residual flux.
If there is no residual flux, one solution is to attach a permanent magnet to the backiron of the exciter fields. If possible, add a north polarity and south polarity magnet radially in line with adjacent fields.