weak battery, bad or corroded connection in wires is where I would start.
Also could be bad spot in starter, or silenoid.
Would be helpful to know if its 6v or been converted to 12v
The other thing I have done is boosted with 12v cabels directly on starter (not battery unless its another 6v vehicle). Make sure its in neutral and don't hold them on long. It will whirl over fast but should start easy if starter is good.
The other thing I usually do if I am having trouble starting my tractors is pull them with a truck to get them started and let them warm up a little so they start easier off the starter.
Not sure how mechanically inclined you are maybe you have already checked this stuff, But good luck!!!