Yeah the shape of the washer is important, sooner or later most KTM engines will start leaking in the CS seal area.
Best bet is to replace the stock CS washer with the Dirt Tricks one, it's made from tooling steel and will keep its domed shape much longer. In a bind - e.g. somewhere on a trip or so - you can also re-dome the washer by putting it in a vice between two differently sized sockets.
Nice to see you back out in the jungle on the new bike! Just take it easy for a while so that the leg has a chance to heal before you abuse it again...