If you got 5,000 hours out of a 1955 engine you were doing something right. Sounds like you need a complete short block assembly. If you are not concerned about originality you might compare the cost of a Cummins engine as opposed to the short block. A thread in here about a year ago or so mentioned a cost of around $12,000 for the installation of the Cummins. A short block would probably be a lot cheaper and if you can get another 5,000 hours on this one, why not go that route? The 1955 is a good, heavy, usable tractor. Would be a shame to scrap it.