I'd try rocking it back an forth with the engine switched off first and see if you can get it to come out. Next step, take off the cover which goes over the clutch. At least you can do this with the engine in the frame. My 350 (same selector and clutch) got stuck in top once and I had to ride it home like that. When I took the cover off I found that the selector bolt had become loose. Put it back together and fine thereafter.
If I remember rightly, should you need to split the crankcases, it can be done leaving all the top end, i.e barrels, cylinder head, cam etc in place.