The 69 140 H-3 I had had steering brakes. Dont see why you couldnt retorfit those parts, but it would be cost and effort that you could probalby just pick up a complete tractor and sell yours.
In 70 or 71 they went with the newer transmssion with drum style brakes. Those are a more prefered tranny, quiter and a ton more of them around as they used them up through the 317.
Steering brakes are kind of worthless other than in snow though on a garden tractor in my opinion.
There is a bit of value to a collector as its the first year and the 12hp, but for someone that wants to use it not look at it, it has less hp and less features and some unique parts that are harder to come by if you need replacements, so in that case no.
BTW I have owned 5 140's over the years, love them, I'm sure I will find another some day.