If you are jumping to the connection stud on the starter switch with a jump from an external battery, you are effectively eliminating the installed battery and the connections from it to the starter.
Remove the nut and washers from the switch that hold the battery cable, and the wire to the amp meter. Clean this connection with 120 grit sand paper and reinstall. Try it.
Remove the battery terminals at the battery and clean them, then replace and tighten. Try it.
Check the voltage of the battery. If 6v it should be 6.3 volts or more.
If 12v it should be 12.6v
If less than that (either) it needs to be charged and retested. If still low the battery is probably bad.
Charging voltage is 7 volts to 7.2 volts on 6 volt systems. It is 14.2 to 14.6 volts on 12v systems. Start a new post, this one is getting old and far back in the pages. (I use the classic view) Jim