I would look at a gear reduction starter.I had an erratic starter in the Lister with the 351W .Sometimes it would act like a bad ground.cleaned and checked every connection.The starter was checked out at a shop whose owner I knew.It tested out good.He gave me a gear reduction starter.What a difference.No more problems.Turns faster and starts like almost on the first turn.
A good shop should be able to help you on this.No I don’t know the part number.I just know he told the girl to look up some numbers and she told him the starter to pull of the shelf.