No reason it shouldn't work on the KTM. Just make sure you have the correct fittingts to tap into an existing line or ports and a one way check valve to keep the oil from draining back into the engine cases when the motor is shut off.
An oil cooler is not something designed after an education in Rocket Science. Its just a simple heat exchanger, like a small radiator. Key issues would be to mount it where it can be protected, from crashes and flying debris. I holed a Porsche oil cooler at the track and it almost cost me a $20,000 motor. One of the easier places I have bolted aftermarket oil coolers is next to the radiators as it receives great airflow. XR 400's have them mounted around the steering head.
Keep in mind they do not have to have a huge surface area to be efficient. Here's a custom KTM cooler I have on my bike and you see the surface area is not overly large.
Custom shapes should be able to be made in most metropolitan areas.
Best o' luck.