Unfortunitely your in for a third [3] tear down. 1st check to make sure that the rear spacer between 1st/2nd gear and rear mainshaft brg is there [ ref. Moss Cat. Pg 122 Item # 45, lower explosion], 2nd I check the detent steel balls for the 1st / 2nd overgear which are installed with springs in the inner gear [ samre Moss cat. Pg. 122 item #42 again lower explosion ]. And when I say check I mean proper orientation of the outer gear in relation to the inner gear. There are only three [3] internal teeth which have the correct detent positions where the balls will locate to operate this function. Look inside the outer gear and you will see the inner grooves [ there are three 1-for 1st,1-for 2nd and a third for the centre netural position.] Last check Lay Shaft bearings [ same Moss Cat. Items # 14 & 24 upper explosion ].
Happy teardown.
Bob