ok the info we need is, are the springs not compressing & rock solid? Or are you just grabbing air when you pull the lever?
Take the cable off & operate the lever with a wrench as a test.
Not sure of old RM but assume that vintage it is
1. a worm drive lever on the LH Side of the engine pushing a pushrod through the centre of the engine.
Or
2. perhaps a lever on the clutch side pulling the clutch toward the cover.
If the former there will be an adjuster on the LHS + there may well be a ball bearing between the pushrod that is wanton to get lost.
If the latter the serration needs to line up as it goes into the cover. Ensure no washer (or radial bearing) is missing from assembly affecting positioning.
Sometimes the pressure plate doesn’t slot down on the basket & needs a jiggle when you put it back together. With the side cover off is the clutch locked up ie: the plates are working (pushed hard against each other by the springs) & the centre doesn’t spin around.
ps will be away so can’t answer further, but someone will pick up the ball.