I've got a 2005 outback with a bad engine and a 1990 legacy with a rusted out body. Can I turn the two cars into one by taking the engine from the '90 and putting it in the '05? The '05 is a turbo and the '90 isn't, if that makes a difference. 

No. Not even possible. 

No, the engine computer management system is totally different.

The block is the same for your 05 for 2.5's from  00-04.   You can use your intake from the 05 on the 00-04 block.   I think Timing gears from the 05 as well.  Others can chime in on exact details.  The block you have can be reused on other 90's cars obviously.  best of luck.  I think you have to mcgiver the pcv on the 00-01 of you are using it on a newer car.

 

OK, I'll abandon that idea. What is the last model year that I can put the '90 motor into? 

98 if you use the intake from the 98, and you will need the exhaust header from a 95. Also if the 98 has EGR you cannot drill and tap that into the 90 head - you will hit a coolant passage. So you have to make other provisions. 

The 2.2 can go into the 99 Outback as well... If 99 you need to use a helicoil for the starter bolt if that Outback has a manual transmission. Also that 90 2.2L needs an intake from a 2.2 that was made from 95 - 98 in order to function and plug into the wiring harness for the car. Preferably one with the EGR intake so you don't get the check engine light. You'll need to bypass the EGR to the head and run it off the plentum instead.