The casting numbers only mean something to the engineers that designed the casting or someone who has access to the internal documentation that tracked those numbers at Ford. Finding one with the identical casting number to the one that you're replacing is the only way to guarantee that you're getting the correct part if all you're doing is looking at the casting number.
Those two casting numbers, C5NE 6090 & C7NN 6090, both end in 6090, which means that it is a cylinder head for an engine, and might mean a bit more, like a 3 cylinder diesel engine, but we can't say for sure without those internal Ford documents.
The first two characters of each number simply tell when the casting was designed, C5 is 1965 and C7 is 1967. The next two characters say which division of Ford it was designed for and which division designed it. NE means that it was designed for the tractor division by the engine division and NN means that it was designed for the tractor division by the tractor division.
If there were more than one casting design made in the same year that would otherwise have the same casting number, then it would have one or more letters as a suffix at the end to designate which particular casting design it was, but again that would only mean something to you if you have access to those internal documents.
When buying used parts, you need to know what model it came off of and look up the part number for the model the used part is coming from and compare it to the part number of the model you intend to use it on. Even then, some newer parts will work on older models but the part numbers won't cross reference.
The current part number for your tractor is D6NN6049A, and tractor-part.com shows several salvage yards having one or more in stock. I suggest you contact them for price quotes, and depending on where you're located, the cost of shipping may be the deciding factor:http://www.tractor-part.com/parts?Number=D6NN6049A