I have a rockewell 83-621 motor that has been converted from 220V to 110V via the wiring diagram. Now I have 2 wire ends that I need to connect to a power cord so I can plug it into the wall outlet. The wiring diagram just say the ends go to the power cord. How do I know which wire would be connected to the positive and which would be to the negative so that I don't electrocute myself or blow something up if I connect them wrong? Thanks.

Well, there is NO "positive" and "negative". With 120v there is a hot and a neutral.
In a wiring diagram this will be referred to as L1 and L2. Same goes for a 240v application, L1 and L2.
You have to determine from the diagram which is the hot and which is the neutral.

Can I just test by trial and error to see which is L1 and L2? What would happen if I wire the hot and neutral wrong and plug it into the outlet?