Wet fouling plugs - 2003 KTM EXC 300

that will be oil vapor from the sump
have a wet and dry compression done to determine engine condition
use plugs with a hotter heat range to burn off the oil and reduce fouling

Have you noticed the car losing water? How is the plug fouling? Does it have oil on the plug or is it wet with water? You could be losing a coil pack too.

and neither you should have to
as it is a 2015 make , why have you not taken it back for a warranty fix
next you would be using a single point plug ( car plug) when it would be better to use a mulit point plug ( anti fouling plug) that is available from good spares shops , motor bike shops and marine shops You mention that it fouls after sitting over night
can you comment of the fouling condition( wet , black, oil ,carbon build up etc ) as the information all points to a problem area.

Sounds like a leaking pressure regulator, since one bad injector would only flood one cylinder.
Is it always getting spark to the plugs ? Keep in mind, once the plugs are fuel fouled they are easily fouled again.

Could be fuel or oil fouling. Need to determine which first off. If it is black sooty fouling it is likely too rich also it too rich may be wet.

If oil fouling it is more likely to be much more material fouling and wet sticky material.

A wet, black and shiny deposit on plug base, electrodes and ceramic insulator tip indicates an oil fouled plug. The condition may be caused by one or more of the following: worn pistons, worn piston rings, worn valves, worn valve guides, worn valve seals, a weak battery or a faulty ignition system.

A dry, fluffy or sooty black deposit indicates an air/fuel mixture that is too rich, engine idling for excessive periods of time and/or enrichener usage for excessive periods of time.

A light brown, glassy deposit indicates an overheated plug. This condition may be accompanied by cracks in the insulator or by erosion of the electrodes and is caused by an air-fuel mixture that is too lean, a hot-running engine, valves not seating or improper ignition timing. The glassy deposit on the spark plug is a conductor when hot and may cause high-speed misfiring. A plug with eroded electrodes, heavy deposits or a cracked insulator must be replaced.

A plug with a white, yellow, tan or rusty brown powdery deposit indicates balanced combustion. Clean off spark plug deposits at regular intervals.

Wet plug simply means you are getting no spark. Could be bad plug, wrong gap, plug wire came off, bad mag coil, or stop wire still grounded. Most times is fouled or burned out plug. Change it out first before major work.

if its injected u may have a stuck injecter if so take it out use a 12 volt battery and pulse the injector in a cleaner or high octane fuel. 108 oct or up

A fouled plug is one where the center electrode is joined to the outside metal bit and allows a dead short or no spark. They are normally completely carboned up or are joined at the gap. They can be cleaned and reset. A flooded plug is where the insulator in the centre is completely wet and allows the spark to track down the side of the insulation to earth --again no spark. Blowing dry with air or wiping dry and they generally go again Lawn mower plugs are a good example

Can you tune the carb or why not try a hotter plug, one that will get the fuel burnt easier. Is it oil that is fouling?

Fouling a spark plug with unleaded fuel and wet fouling a spark plug with ethanol fuel is more common than you would think. Take out the old plug. Test for spark with a new plug. Then put in the new plug.