Hi. I have a 1990 Legacy with low mileage, but some external and electrical issues.

For instance, I need to replace the rear wiper arm; mine is cracked in the middle. For that specifically, I've found a bunch of car-part for second gen, but not many, or any for 1st.

There are a bunch of other little things like this.

How can I determine which parts might just fit, or be fittable on the older gen?

Yards are the best, if you have ones you can walk through.  I know it's winter up there so not the best time.  www.car-part.com is a good source as well.  

The rear wiper arm is pretty standard.  I have several if you need one.

Larry

parts from 90-94 should work fine - "should" being the operative word. there are some minor cosmetic differences in those years, but for the most part they are very close..

might even be able to make 95+ work.. kind of depends on how long the arm and the blade are... find the right combo and you will be fine.

i guess my concern would be does the motor work/turn the shaft? if not used regularly, things tend to seize up... the 1990 I had years ago had that problem... I was able to take things apart, clean it up, grease it up and get it working again, but it was a bit of a chore. i seem to recall being terrified of breaking the glass tho, where the shaft passes thru..

as for other parts.. it really kind of depends on what it is you need... there are a lot of things that can be swapped over between 1st & 2nd gen, but there are a lot of things that cant, too.. just depends on what specifically is needed.