Mine broke exactly as yours did. Had it machined, drilled, pinned, welded, and faced (trued face of flange to axis of axle) 3 years ago. Cost < $300 but I did the disassembly and reassembly. It no longer gets 700~800 pound logs chained to the bucket it but works well for everything else i.e snow removal, bucket full of wood, carrying saws & tools into the wood lot,... everything but maxing out the FEL capacity. I've accumulated some tools and changed some technique so the 3-point does the heavy lifting now.
I have the prints and specs for the original axle if you're considering having a new axle made. PM me and I'll get them to you.
The replacement axle Ford introduced as a cure for this issue in 1979 ~ 1980 (I could never pinpoint the time or serial number the fix was implemented) is oversized in diameter. It is not a one part swap. It involves several components and is quite pricey!
If it's a strong runner and mechanically sound other than the axle you'll have no trouble selling it as a parts tractor, but $3500 is steep in that condition. If repaired (it looks in better shape than mine) I'd guess it would bring in the $6~7k range very easily.