Yeah, a 70kg person is pretty much the target market for a YZ100.
I rode one once, as a kid - as worn out as it was, it was a great fun bike to ride. Roger Harvey actually had a couple of pages in an ADB in the late 1980s, talking about how great his worn out YZ100J was, and I agreeed wit everything he said: it needs to be flogged to work properly; its still slow by the standards of a 125 of the same age; it makes you feel like a god when it you're riding it.