Belts or magazines....
Each system has it's advantages. Magazines obviously allow you to pass ammunition from the squad to the automatic rifleman, but only if your SAW/LSW uses the same magazines as your rifle. Changing magazines will decrease the volume of fire that you can provide in a given period of time. The 5-10 seconds it would take your gunner to change magazines could allow the enemy to get his head up and bring fire on your troops while they are exposed and moving.
Belts allow you to sustain your volume of fire for a longer time (the M249 SAW uses 200 round belts), but eliminates the massing of magazines to the automatic rifleman to keep him in the fight when ammunition runs low. The US Army has tried to have the best of both worlds by having the M249 fire ammunition from belts or magazines. In reality the magazine function has never worked well on the M249. If you don't hold the magazine into the forward part of the mag well with your left hand, the weapon will quickly jam.
I have seen photos of the MAG 58 with the ELCAN mounted. The US Army version (M240B) has a Picatinny rail on the receiver cover for mounting optics. My personal opinion is that optics on a machine gun are a waste. Machine guns vibrate too much to make use of an optical sight practical while firing. Machine guns are area weapons, not sniper rifles (the M2 .50 in the sniper role is the exception). Much more practical items to issue with a GP machine gun would be a mortar sight, aiming poles, M2 compass and graphic firing tables for the round the GPMG is chambered for to allow the platoon to use them as indirect fire assests when the terrain calls for it.