Just because no one responded doesn't mean we don't care...
FarmTrac was never a very well known tractor (chinese and were sold through Northern Tool, Home Depot and another store). They were built cheap and sold cheap...very poor quality.
And, no dealer network that I know of...so the owners were pretty much on their own.
There's one on Craigslist (York PA area) the guy can't give away.
Now, back to your question...I don't know the size...good chance you may not get an answer here. If you have a FarmTrac, your best bet may be pulling one on the other bolts and take it to a hardware store to size it.
That's about all I can offer...