John deere

I picked up a 1966 5020 john deere tractor going to the baltimore port.
The guy who loaded the thing assured me that the tractor runs and all it needs a jump or new batteris,he used a forklift.
Went home swapped the batteries,turned the key on and hit the start button,it cranked couple times than the starter just kept spinning even without me hitting the start button.
Unpluged the positive wire and when i try to plug it back it sparks really bad and the starter starts to spin non stop.
Anybody knows what im doing wrong or what i need to check?

 

12 v hooked in series,its a 24 v system

 

The john deere dealer told me its a faulty solenoid,can u explain why the starter doesnt engage

 

I unhooked the wire to selenoid,starter still spins non stop,hit it with a hammer.nothing helped. I dont care about the buyer overseas,i just wanna drop it running at the port.
Can i replace the selenoid or do i have to replace the whole starter

 

Can u please tell me how?it looks like the selenoid is built into it