can i swap a 1991 legacy 2.2 into a 1996 legacy with a 2.2

It's possible, use your old intake on the new engine.  Your 96 most likely has a single port y pipe and the 91 has a dual port y pipe so you'll need a single port y pipe.  The other problem is the EGR Configuration. you need to stay in the same configuration as the 96.

the 91 does not have EGR,

the 96 MAY have EGR.

 

so if the 96 car needs to see that you will have to do a little work to make it happen.

engine wiring harness is also different and would need to be swapped.

 

91 connects near the under-hood fuse box.

96 connects at the passenger side of the tranny.

 

If you have emissions testing it would be best to go with a more compatible year engine (especially if your car has EGR)

If you dont have emissions testing, and can live with a constant CEL, then yeah, the 91 is do-able with a little bit of effort.