You will have to replace all the gears and all the rails. The pieces are all available separately from numerous sources.
This job will stress a DIYer. It is a 25 hour job requiring the removal of the front timing cover and some amount of the pan. I have heard of techs who pull the motor to do the job. Lots of things to go wrong.
Bosch Master, ASE Master, L1
33 years MB technician