Do without it then. If you fill the radiator with the car level or even with the front raised some and keep filling it until the side pipes are as full as you can get them, cap it and then lift the rear, and you shouldn't have any air in the rad. With an expansion tank the highest point, fill it until it bubbles no more and then start it up and keep filling it. With a bleed at the hot water exit from the engine (high point of the hot side), you should be able to get it all. The expansion tank is going to need a drain back to the water pump. The NA water pump might not have a fitting for that and you might have to tee into the "oil cooler" return or the heater loop return.

https://www.subaruparts.com/v-2002-s...and-components
There's the expansion tank. It shows 2 radiator caps, #13. The round one is for the rad and the one with ears is for the expansion tank.

Ed