It must have a fully charged and healthy battery first of all. If in doubt replace it or give the engine 15 min or so at 10 volts or less to bring the battery up. If the problem still persists it's possible the charging circuit is corrupt.
As mentioned above, transformers are an all important factor in getting these older engines to run and following MTH's list of acceptable transformers is a good place to start.
Hope you're making progress and get the little guy running.
