Keep Coding

Since there isn't a NOT logical operator in mips assembly, how can one "not" the contents of a register?

Load -1 into another register, then XOR the registers.
https://www.cs.umd.edu/class/sum2003/cmsc311/Notes/Mips/bitwise.html

You can use nor and the zero-register to fit everything into one instruction:
nor $<dest_reg> $<in_reg> $zero
Another approach would be using xor and -1 as this is encoded as 1111...1111 in 2s-component.
li $<help_reg>, -1 # load -1 into help-register
xor $<dest_reg>, $<in_reg>, $<help_reg> # actual not-operation