I might enhance Traditional Farmers reply, if my memory holds up. I "retired" from an AC dealer about the time AC died. The 5040 and 5050 looked very similar. The 5040 was a Fiat design, manufactured in Romania under license. The 5050 was made by Fiat in Italy. A little different in size, and the fit and finish appeared much nicer on the 5050. I don't recall any serious problems with either. I recall a Morelli starter which didn't like the cold. Any Lucas starter, likewise.
Fast forward- a few years back, I was looking to find something easier on the old body than an antique(confession). I found, and bought cheaply, a NH TT60A. I haven't done a side by side comparison, but it's just an old Fiat design with electrical and sheet metal updates. I went to change the front wheel tread, and couldn't get the tie rod to move. This, on a tractor with less than 500 hours! I pounded and heated a little, but decided to research before putting a lot of expense/effort into it. I got a complete new assembly for less than $70. I don't know if the new parts would fit the older tractors. But, given that they are made and sold all over the world, and have been for so long, there has to a much better chance of getting parts than there would be for something totally discontinued.