I was around a lot of dirt bikes in my early-to-late teens (mid-70s - early 80s) - I don't believe that I have ever seen a left leg kickstart. Although your question made me "google" - and it appears that was popular during my riding time with Euro bikes. If I remember correctly - the only bikes that I can remember are Honda, Yamaha, and Kawasaki.

I raced Kawasaki (KX400), but pretty much sucked at it and got tired of the ER visits, or maybe it was my parents that got tired of the medical bills. I haven't ridden in many a year, but you are right - once you hit the power band you had better be up on the handlebars. My cousin was VERY good (pro good), me not-so-much - I think he was fearless and me, not-so-much.

Note: After going back and reading IrishGuy's post about the Bultaco - I do remember those bikes, but I never rode one and had no recollection of them being left kick. Once again - went to google.