I will take a look to see if I can remove the lead screw without having to remove the saddle. Not clear from the parts diagram how the lead screw is removed or what needs to be taken off to get it out.
Again, only if it is not too much work! And certainly do not take the time to disassemble the seat and bearing assembly (# 414, 415, 416, & 418).
If it is anything like mine, the screw assembly should come out the front. I think you will have to remove the hand crank (#423) to get to the mounting screws (#417). Then you should be able to unthread it from the nut (#437) and take it out.
If you do get the screw out, the critical measurement (I think) are:
A : Overall length
B : Length of threaded portion of shaft
C : Threads to shoulder for bearing (or from end to bearing shoulder
D : Shoulder to hand crank end of shaft
E : Diameter of bearing shaft
F : Outer Diameter of threads
And, again, please do not go to very much effort to get these dimensions!!