When the forks were made, adding air pressure was one of the "back then" tuning adjustments. KDX forks since 1989 have switched to just letting built up air pressure out of the forks. Get the front wheel off the ground and bleed the air. Go ride, then bleed the air again after riding. "Nowadays" it is just a "bleed the air befoe and after riding" method.
Cannon Racecraft might make custom fork springs for your bike. If you don't want to get "current thinking" stiffness fork springs, then go with a higher oil level (within the manual recommendations) or try modifying the damper rod valving or get Race Tech Gold Valve Emulators.
Good lcuk with it.