We got a nuffield 460 and its a great tractor and got plenty of power, never had a problem with spares for it. In fact just bought an engine rebuild kit and found a new block as cracked the old one last winter. We used it on a big road making compressor for running the rivit guns and squeezer when doing william and it never missed a beat. Quicker on the road than a major as well.
Jack vickery has one for sale atm
http://www.tractiontalkforum.com/showthread.php?t=20199