Hmm how much lighter I wonder? You may be very surprised at the difference in the bare frames with only forks and shockers and wheels fitted.
By all means remove the SL's steel guards and cut off all the road going brackets , add a set of CR125 forks (That are heavier than the SL's standard ones !) a good pair of rear shockers and CR125 wheels (fit very easily just adjust spacers). leave the standard tank and mull over what to do with the fat arsed standard seat, which on mine I leave... Resulting difference in weight between the two rollers is roughly 2.7 kilograms... the biggest weight difference is the four stroke single overhead camshaft lump as opposed to the trim little Cr125 two stroker.
Friend I have done what you would like to do, and well it was great fun! but in reality not really any difference except I rooted a perfectly good CR125 frame. Sadly that rolling frame less engine was totally obliterated in a shed fire here, worst part was the wheels ,tank ,seat, guards and sidecovers were fitted. Cheers enjoy the challenge