I got back into riding after a long layoff and started with a 91 WR500. I'd never even owned a 250 before that. My last bike before the 500 was an 81 Yamaha IT175 I bought new in 81. I didn't have any problem adapting to the 500. As mentioned, it has a soft lowend. I think that's got more to do with no powervalve than anything else, though. At any rate, the bike was extremely reliable, stable and fun to ride. Being air cooled and no power valve means less to go wrong but the cooling fins ring pretty good.
The YZ is a bit of different animal. Closer ratio tranny and smaller tank at least. Is that a 4 speed or 5? Some of the older 490's had only 4 speed trannys. Not that you needed any more gears for MXing.
To get rid of the pinging, send the head off to Noleen. They did a $60 or so head job on mine and it ran great after that. No more self destructing noises or pinging. I never touched the jetting from stock and I ran pump premium. I haven't heard anything from Noleen for a while. Are they still in business? Their phone number is/was 619-246-5000 and they're on the left coast.