Hold off on the points. Was the motor dry when you tried starting it. Meaning, was the tractor located in an area where damp air could reach it? Had it rained lately? In a lot of cases, a sluggish start especially in a damp environment, leans towards a coil which is getting ready to go. If external coil, yes, an automotive coil will work. All standard Coils perform the same function, they take a low primary voltage and jump it up to a high secondary voltage.
Cedar Mill Bumper and Hitch