An additional muffler won't help. They already have mufflers.
If the Cannon plugs are all attached, the gauges clear and the unit isn't missing equipment like the control/output cubicles, it should only need filters and cleanup of the fuel system.
That they probably have been sitting on the DRMO/GL lot for a few months is the limit of the damage. Mine needed fuel system cleaning plus gauge/meter servicing. All but the frequency meter were salvageable. This included taking the percent rated current meter apart and re-gluing the damping magnet as water intrusion caused it to debond, pinning the damping vane and preventing movement. They are sealed but you can take them apart and clean/adjust if you know how.
The oil pressure gauge was dirty and leaked rain water. Plus a spider had nested inside. The fuel gauge had corrosion too, it was easily opened for cleaning and repainting.
The Nylon outer covering on the exposed wire was gone, this was replaced by clear heat shrink to protect the fiberglass. Then the wiring harness was loomed with corrugated split loom.
The most work was the fuel system. Tank had about two dozen rust holes and full of rust. The filler cap and strainer basket was desoldered, then rivets drilled for complete removal to allow complete access . Phosphoric acid eventually removed all rust, holes welded and interior coated with POR Universal Tank Sealant. Like new and should never rust.
The injection pump was gummed and inoperative. With good dexterity, the control collar lever assembly was removed to clean properly. Then a few shots of Berryman B12 inside the pump cleaned the rest. No need for retiming the injection pump but still takes a few hours.
This MEP-002A is rock steady at 60.5 Hz under a 3 kW load and jumps to 61 Hz when that load is removed. It has driven a stick welder at 200 amperes output (roughly 35 volts arc voltage) without blowing any smoke.
If you have heating by #2 fuel oil, the 002/003As are perfect. Noise is best controlled by enclosing. Just baffle the air inlet and exhaust. Some line the enclosure with lead sheet but regular drywall (5/8", fire rated) is sufficient as the noise is lower frequency once warmed up. Lead only stops the initial diesel clatter.