What can I substitute for hot Korean chili oil in this pork noodle recipe?

You can always just make your own Korean chili oil. For example, here's The Woks of Life formula for chili oil:

1½ cups oil (ideally a vegetable, peanut, or grapeseed oil…light olive oil is fine, but it has a tendency to set in the fridge)
5 star anise
1 cinnamon stick, preferably cassia cinnamon
2 bay leaves
3 tablespoons Sichuan peppercorns
¾ cup Asian crushed red pepper flakes (Sichuan chili flakes are the best)
1 – 1½ teaspoons salt (to taste)

Heat the oil, star anise, cinnamon stick, bay leaves, and Sichuan peppercorns in a small saucepan over medium high heat. When the oil starts to bubble slightly, turn the heat down to medium.

I would use any asian Chili oil. My experience is that they are pretty much the same. I have access to an asian supermarket and all the chili oils are in the same place in the market as well as the hot sauces. Chili oil is very spicy. If not available us Siracha near the end of the saute process instead.

There are just a couple of things that separate Korean hot chili oil from other types of chili oil. Usually, it's made with hot oil, with dried hot pepper flakes added. The oil gets infused with the chili oils and flavors, and then is strained so you just have the flavored oil.

Korean hot chili oil is different in a few ways.

1. The pepper used, specifically, is the Korean gochu pepper. I think, while preferred for completely authentic product, this is probably the least critical aspect, as many types of dried peppers are called into service across cuisines as substitutes for one another.

2. The pepper used is ground, instead of whole or crushed/flakes.

3. The oil is additionally flavored with garlic and ginger.

I think you have a couple options here:

Make a batch of your own, and store it in a jar. Method/recipe is here:

Jihye Change: How To Make Chili Oil (Korean Style)

Or use a standard Chinese chili oil, but heat it up and then add some fresh grated ginger and crushed garlic when it is hot, and give it a couple of swirls before tossing any food into it.