You will have to remoe the gears to replace the rails. The engine does not have to move but beginning reference points should be established just in case something doesn't turn out.
The tentioner is no big deal. After the motor has been turned by hand (in the proper direction) the tention is on the right side of the motor. If you spin it backwards (which you shouldn't do) all the slack will not be where the tentioner is an getting it in place will be more difficult. A large screw driver pushing the tentioning rail takes care of it in any case.
Bosch Master, ASE Master, L1
33 years MB technician