The nut piece that your saying is busted off originally was used for the recoil starter that was standard equipment on the super versa-matic. 9 out of 10 tractors they are missing from because the recoil starter would fail the owner would remove it trash it then use a rope. The super versa was introduced in 1954 an continued till at least 1960 with only slight changes made throughout production I have a very early model as well as one of the last produced can't hardly tell them apart. These are very handy units an a lot of fun to work have tended a garden several times using one from plowing to cultivating also used a lot to push snow You can't beat these little kohlers they knew what they was doing when they built them. Looks like you have a pretty complete unit only missing the belt guard from what I can see
I usually determine the approximate year by the shade of green and yellow used and by the model number of the machine.
they all look pretty similar though.