Poncho is very Correct! Every-time the engine "Flexes" you are going to Slip the belt off, or stretch it and blow it! Even gear driven will cause damage.
I believe the alternator will charge in either direction, however, before doing so, I would replace the brushes and burnish the shaft pick up as It would be used to turning the other way (brushes slanted at that angle) as opposed to the new direction.
What type of engine, and What type of Vehicle are we Dealing with here? More info might help you find an alternative mount.
If it's some Type exotic Non Road vehicle, you may want to consider something like a drive shaft, or PTO driven alternator if no room exists at all in the engine compartment, But mounting and balancing The drive pulley will hit you in the pocketbook very heavy.