If you can rebuild an engine, you should be able to rebuild a gear box with a bit of paitence.
Just take it slow, and take note of where it all goes when you take it apart. Having somthing like a Gregorys manual to see what it sould look like before you strip it is a good idea.
From tinkering with a gear box myself, i found it wasnt that hard as long as you kept things standard and took your time. Its just fiddly and time consuming. remeber to keep the work area clean!
Keep in mind, once you have it apart, you might find that to replace one syncro, it may be better in the long run to replace more thn just that syncro. Just depends waht it all looks like once its apart.