When you say stiff, do you mean its crunching into gear or just loads of resistance?
As already mentioned:
grease the linkage points, esp the ball/socket on the gearstick itself, just lift the rubber gaitors.
The release arm on the clutch can seize, just where it goes into the gearbox bell housing.
Check the 'seating' and housing of the slave cylinder. They are plastic and flimsy retaining clips.
Someone on here said about 'packing out' the push rod, place a 5mm packer into the socket of the release arm.
Someone else drilled the bolt holes on the slave cylinder mounting bracket itself, this allows movement of the slave cylinder to the left.
Maybe an gear oil change might do the trick, although with the OP at 45k I am not convinced it is the oil.
MTF94 is the same viscosity as 10/40w engine oil, only it has friction modifiers in it (and other stuff).
Some mechanics cut corners and just fill with any old oil.
If all that fails, then come back and wll talk fork selectors and gearbox internals