How low do you want to go? I'm thinking 1-2 inches is as low as you can go with a different set of springs. If you want to go lower a new wheel would be in order.
But the sidewall on the 21" tire is shorter than the 17", so the tire diameter wont be a 4" difference....If your stock wheel is a 17" and you went to a 21" that's a 4" height difference but from center of the wheel only 2" difference. I think 2" drop would get you pretty close to stock stance.If stock was 18" that's a 3" difference so 1.5" drop but you May want to search the forum for people who have lowered their bike 2", because 2" lower is going to affect the travel in the front.
With that minimal of difference you may want to leave well enough alone.120/70/21 = 27.67 diameter
130/80/17 = 25.2 diameter
Your front end should slightly more than an inch higher than stock.