Checking user input for boxing sim engine
Assuming that each Boxer object has a .name attribute…

BOXERS = [None, JS, MJ, SR]
for i in range(1, len(BOXERS)):
    print("%d) %s" % (i, BOXERS[i].name))
boxer1 = BOXERS[int(input("Choose a fighter: "))]
boxer2 = BOXERS[int(input("Choose his opponent: "))]


You want to generate the menu using the names already in the boxer objects, and retrieve the result with an array lookup.

Note that with this solution, you'll get an IndexError if the user picks an invalid entry.  You should decide how such errors should be treated, since your original code also lacked error handling.