Is the electrical system 6 volt, or 12? 6-volt ignition systems are bad about having starting problems, especially if the cranking speed gets a little low. Check the voltage available at the + terminal of the coil during cranking. It should be a steady 4.5 volts or more on a 6 volt system, or at least 9 volts on a 12V coil. Repair connections and/or grounds as required. Don't forget to check the voltage drop across the ignition switch. It should be less than 2/10 volt from the battery supply wire at the ignition switch to the ignition coil + terminal.
Jerry
