You do need a main disconnect, and a ground rod, and to isolate the neutral bar, and to add a ground bar, at this remote sub-panel.
I would replace that panel with a typical 100A main-breaker panel. Much easier and you will not lose the two breaker spaces by using a backfeed breaker. If you do backfeed the breaker must have a retention device installed.
If I were you I'd try to re-wire the machine for 240V, NO question. If you insist on 120V then you need 10/2 cable and a 5-30R receptacle and 5-30P plug for the cord. That is assuming the cord is already the correct size.
If it is already wired for 120V why would you need to change it?