I have an 01 ej251 with no inner or outer timing belt cover. I would like to know the interchangeability of timing belt covers of 1996 plus ej22 and ej25 sohc engines.  

you need the long 2 piece cover for your 01.  There is the driver side small cover, but the middle and passenger side is all one piece.

Which (years) timing belt covers will fit the 01 ej251 engine.

2000-2004 for sure

pretty sure the EJ22 covers will fit the 2000+ EJ25. I can confirm this this afternoon. If so, I've got some covers I can sell cheap.

Yes I'm curious whether the 1996 thru 1999 ej22 timing belt covers will fit the 2000 thru 2004 ej25 sohs  Like previous poster said the ej25sohc the passenger side middle and passenger side is all one piece. Where on the ej22 1996 to 1999 has three pieces.I think the 1999 plus ej22 phase 2 has the two piece timing belt cover. I don't know why Subaru changed from three piece to two piece. However it must be a fitment issue of some kind.