Oh dear...... 
This is gonna be my first bash at replacing spokes. So is there rocket science involved in correct lengths? I just measured from end to end, 120mm.
120mm is very short for 20" rims .
When you say skinny wheels im assuming they are 20 x 1 3/8 ths rims
In which case i would say it's more likely to be nearer to 220mm , i built some with 215mm spokes.
Tell me the size of the wheels and the hubs you want to use and i'll do my best to help on the correct spokes
Sean (Stidds) has done a spoke calculator and wheel build thread in the Tech and resto stickies
