As for fixing it, its a gearbox off job, then dismantle the inners of the gearbox to replace the synchromesh (which probably costs a bit for the part on its own). If you saw the amount of cogs and pieces to remove it would scare anyone.
@4thgenphil might have a pic handy to do just that?
If it was this, the lower cost option is to buy a replacement gearbox. But...
Clunky might not be that bad, its when you get the reall grind and crunch changing from one gear to another (with the clutch depressed like a normal gear change).
Another solution to see if it helps the gearbox is something like Amsoil MTF:
http://www.opieoils.co.uk/p-1051-amsoil ... d-mtf.aspx
But I'd try to source the noise first, assuming it is the gearbox making it (check those driveshafts as suggested), check the noise when you press down the clutch to see if its the release bearing and then start thinking about the gearbox issues.