Ok, back with more info.
The "new" 690 engine was first used in the Husqvarna 701 and the KTM 690 Duke, and the KTM 690 Enduro was the last bike to use the new engine. Do your own research to determine the exact years that the "new" engine was used in the various versions of KTM and Husqvarna bikes that used the 690 / 701 engine.
The "new" engine uses a valve train that is significantly different from the valve train of the previous version of the engine. The old engine had problems with the rocker arm bearings failing, and the staking used to retain the rocker shafts in the rocker arms was another failure point. These aforementioned failure points have been well documented and the root causes are a combination of design errors and poor implementation of what was a bad design to begin with. There was a thread on Advrider which discussed these problems, but that thread just
"disappeared". Some of us know why that happened. You can make your own guesses as to why, but if you suspected KTM USA you might be on the right track ...
The smartest way to own a KTM 690 or Husqvarna 701 is to buy a new bike and make sure to pay for an extended warranty and then SELL the bike before the extended warranty expires. The 690 / 701 is a very complex engine which has a lot of potential failure points and some failures can cost serious amounts of money to fix. The KTM 690 is a fun bike to ride when everything is working properly, but my experience with these bikes has been that you won't get many months of service before some problem interrupts your ability to ride the bike. And it will cost serious money to fix the problem, more often than not.
A used 690 made before 2019 would be a bad risk. If you choose to buy such a bike, be ready for trouble - because the bike IS going to have trouble and the only question is when.
It's possible to mitigate risk relative to the rocker arm bearing and shaft retention problems in the "old" engine by installing new rocker arms every 10,000 miles of use. But think about that for a minute : this should not be necessary in a properly designed engine. I suggest that anyone who is considering buying a used 690 should think long and hard about the downside risks before committing to buy the bike. KTM USA has consistently refused to take responsibility for selling defective product, and if you buy a used 690 which has the old engine design, you are not
going to get help from the dealer or the importer when the problems happen. Do yourself a favor and buy a Suzuki DR650 instead, and be happy with how it performs, given that it will be utterly reliable and not have expensive failures like a KTM 690 or Husqvarna 701 will probably have.