Most likely, most Subaru heads/blocks are interchangeable as long as the coolant passages line up. The passages on the FB25 motor are really small.
The FA20 uses a unique piston design due to the DI system, so something would have to be made to drop into the FA25.
The bigger issue may be the bellhousing on the transmission is the wrong shape, the FB25 is only used in AWD applications and is meant to mate to a Subaru transmission. However, you can get a new bellhousing casted fairly easily with enough money, there is a guy on NASIOC that does them.
Here is another thread on the topic:
http://www.ft86club.com/forums/showthread.php?t=2158