TOPIC: Getting a 1977 MFG Caprice going

Just picked up this 1977 17’ MFG Caprice for $700, needs a little work but nothing too major as far as i can see. Main thing it needs right now is a starter. Its got the mercruiser 120/140 4 cylinder. I found a parts diagram online but there are a couple different part numbers, 50-69863A 1, 50-59799, and a few others too. How do i find out which one i need?