I have also place a same sort of question on this forum so i would be interested to know how you get on, my clutch went on me, so i replaced the clutch and new thrust bearing, could not bleed clutch so i replaced slave cylinder twice and the master cylinder, then i had drive for minutes then the pedal went hard, took box back out to find the fork knackered. put back together it all worked fine until i put props back on & lowered to ground, now have same problem as you, will not go into gear with engine running but will with engine switched off. i can then go into gear holding clutch down & start engine then works but cannot change gear. if i get any help i will let you know hopefully it is same fault!!!!