Here's the reason I laced up a 17" front wheel. If/when I take an extended trip on my KLR, instead of carrying two different sized spare tires, I could carry one spare tire (4.50"x17") and use it for either the front or rear.
The tire is a 120/90/17 Kenda K761 mounted backwards. A 21" front tire has an approximate diameter of 27 inches while the Kenda on a 17" rim has an approximate diameter of 26 inches (taller sidewall). You might want to build and install the front wheel first then decide how much to lower the rear.
Let me clarify things a bit. I'll soon be 55. Never had a broken bone and have no desire to have any. With a 17" front wheel my KLR still rides within my boundaries (read....skills). I have absolutely no desire to scrape pegs on either my KLR or my Strom.. With that said.....I like the look of a 17" rim on the front of the KLR.yeah, the 17" front wheel looks very cool, simplifies the spare tube thing, and it works reasonably well if you don't ask too much from it.
At casual pace you would be fine.