There were some unique things about those KZ-400s (and I don't mean the 2 stroke ones). Like the constant spark (no distributor design).
My first 'real' bike was when I was 15 and got a 1978 750xs Yamaha box of parts project.
It took about 6 months to put together. My feet could not both touch the ground at the same time. But WOW did that bike haul. No drivers license, no plate, no registration, no insurance, no helmet, no brains.
I rode that all over town and from Middlebury to Howe on several occasions.
REEEAL lucky to never get it impounded. (or just too fast).