To get the pto disengaged you might be able to stick a small prybar down through the top plug hole and slide the lower gear 6 forward, and out of mesh.
There's no easy way to get in there - the L355ss can be split at the back of the trans housing (see pdf) The pto shifter parts in plain view then. WSM pages S.2-15, 16, 17 sorta point at it but no clear pic of the fork . Good luck, Dick B