The S2k transmission is not terribly different than other synchromesh transmissions. It has some neat features that make it very compact, like having the synchro hubs on both input and intermediate shaft. If you take off the bell and tail housing, the actual transmission itself is incredibly compact. There are some delicate aspects to it, and some parts are incredibly tedious, particularly setting the output shaft pre-load, and input shaft thrust. If you're careful and methodical and you have the time, rebuilding a transmission is not some type of black magic. Everything is actually very straightforward. You can get enough parts to slap one together by taking the used transmissions from people you help swap out replacements

The typical culprits are second and third gear, although fourth is also finicky. 1st 5th and 6th typically don't wear out the synchros. These transmissions are expensive to rebuild, not because of parts, but because of the time involved. If you have the time, it will cost you the same to rebuild it as it will to buy a good replacement. ~$1,000 about half of that if you harvest parts.