There are two likely faults with the up-pipe. Often as said earlier the heat shield breaks off and falls down the pipe making an annoying but not unroadworthy metallic rattle sound.
The other less common fault is a split in the flexible join caused usually by the exhaust not being mounted securely and fatiguing the join. This make a foof-foof-foof noise like an old Volkswagen Beetle. Usually worst when cold and often will be less noticeable as the car warms up.

My experience with replacement non genuine up-pipes and headers has not been great in terms of easy fitment. You may be able to source a genuine Subaru secondhand pipe and change it relatively easily, of try your luck with replacement stainless steel headers. If you decide to go with the headers my advice is get the seller to organise fitting for you.