The first thing I would do (if you haven't done it yet) is replace the glow plug, and use fresh fuel. That will at least eliminate some variables.
If the car still won't get up to top rpm, then I would look at the high speed needle. Is it blowing a lot of smoke at WOT (wide open throttle)? Leaning the main needle down will get it up to max rpm faster, and with less smoke. Just remember to tune a little at a time, and smoke means 'rich', and 'rich' is a good thing for these engine.
If this is ok, then I would look at the clutch and drivetrain to see if anything is bound up.
Hope some of this helps.