1964 Volkswagen Beetle Difficulty changing gears

My 64 model VW beetle started to become difficult in its gear changes some time ago. When changing down it is difficult to get it into gear but sustained pressure eventually gets it in. There is a degree of clunking as I accelerate or back off suggesting a looseness in the drive train. Depressing the clutch fully when starting off and putting into gear makes the car rock forward slightly.
It was suggested that the clutch pressure plate may be at fault. Is this likely to be the case or does it appear to be a gearbox issue. Once in gear there are no noises or roughness in its running and it behaves normally. There are no oil leaks in the gearbox. I would appreciate your advice on this. Many thanks