As ID said, remove the old and other solids, replace and check the weight with the existing drywall and on center spacing: Residential ENERGYSmart Library
Cover the attic side of knee walls with house wrap to prevent wind-washing and convective loops in the wall batts: http://www.simplesavings.coop/simplesavings/SIMPLESAVINGS%20knee%20walls.pdf
Stuff small plastic garbage bags with f.g. to air seal under (between joist bays) and over (between rafter bays) the knee wall- tight to the baffles at the sloped ceilings. Use foam electrical outlet covers to air seal the heat loss there.
Wear a mask, gloves, long sleeves and goggles around the mouse feces as it can cause major health problems.