you wont want to fit a 79 PE swing arm, it will be a step backwards. go for the RM 125 T or a 80/81 PE 175 swingarm. the 80/81 PE swing arms are the same and are both alloy. 79 PE swing arm is not alloy.
i dont have any 175 swing arms i can measure sorry.
RM 125 T shocks are about 414mm long or somewhere approximatley around 16.5" 80 and 81 PE 175 shocks are 400mm or 15.75" long and have less travel than the RM shocks
these bikes have the same swing arm pivot bolt so that might help you
Part # = 61211-40201
Part Description = SHAFT,REAR SWING ARM
RM100N 1979
RM100T 1980
RM100X 1981
RM125-N 1979
RM125-T 1980
unless you need quick release wheel i would got for a RM swingarm
RM 100/125 T swing arms are different part numbers but i couldnt tell you what the difference is