I'd say for the third verse to go back to the original:

3. Johnson fed him on a rotten stump and lumps of yellow clay.
He fed him on some wooden pegs; that were his oats and hay.
That mule would chaw, with his iron jaw, a pair o' old dirty socks.
He'd wink his eye like he had some pie, with his mouth as full of rocks. [CHORUS]

On the 4th verse, it just need to rhyme with "alone."

4. That mule chewed some old bed quilts, also a wooden stool.
Johnson he concluded then that old mule were a fool.
"Johnson thought that old mule had chawed some tobacco"
You'd better bet that Johnson let that old gray mule alone.

Or...

4. That mule chewed some old bed quilts, also a wooden stool.
Johnson he concluded then that old mule were a fool.
"Johnson thought that old mule would kick from head to toe"
You'd better bet that Johnson let that old gray mule alone.

Thanks,

Richie