You've got the standard 3-speed transmission use from 48-54. It's called an SM-318. Some parts are unobtanium like the 2nd gear synchros, but eBay has various parts from time to time.
Unless the gears are pitted, they're likely OK. A common rebuild is to replace bearings. The ball bearings would have numbers on them that can be crossed to modern numbers, and should be obtainable at any bearing shop. There's no "2nd gear cluster" but there is a slider that has 2nd and 3rd synchros and that the 1st gear slides on.
HERE's an illustration of your transmission. If you browse thru section of the Service Manual (select previous or next page on the link) you'll get a better understanding of what's in there.
[on edit] Do a google search for "sm318 transmission parts" and pick your poison.
Looks like the other guys have gotten their licks in before me. Good info.
