Clutch

do you mean the (shaft the push rod passes thru) mainshaft? or the (splined part the chain sprocket mounts on) sleeve gear?

is it in gear (simple but just checking) once the diaphram and circlip are fitted an spring compresser released the spring will compress the stack of plates and lock them to provide drive (hopefully it should the book says it does but clutches dont read an often dont drive either) untill the push rod in the mainshaft pushes the spring off the stack of plates.

is the clutch mounted correctly on the mainshaft so the outer is free to spin but lined up with the engine sproket and the small circlip holding it in possition has not failled. there is a spacer next to the clip with a small recess that should be fitted facing the clip to pass over it and stop it being pushed out of its groove

is the outer basket bearing ok? and the mainshaft bearing nad sleeve gear bush? the out can move maybe 1/16 of an inch (1.5mm) raidialy (rocking side to side) at the primary sprocket teeth even with new bearing but no more. there should be no play (or a few thou at most) in the mainshaft an sleeve gear

is the diaphram inner face touching the mainshaft end? extra aftermarket oil seal can be to wide an jam the assembly.

did it work before ? if so why was it taken apart? you may have the wrong parts if it was a box of odds an ends type "project" (been there done that)

hope this helps sorry if its all a bit simple but i am not sure what you mean by saying you have not assembled the gearbox just the layshaft. The layshaft is the lower shaft inside the box an the clutch fits on the upper mainshaft that is insde the primary drive case.