Will a 04 Forester motor work in a 2000 outback?

Yes, it will. You will need to put the original cogged crank sprocket on it, as well as the original left side cam gear. You may also want to put your intake manifold on it as well. And while you are in there new idlers, water pump, tensioner and belt. also cam seals and crank seals.  

Should he pull the oil pump and check for loose screws?

I've never found them loose on the 2000 + 2.5 engines but while you are in there its good piece of mind to check.

Also, 2000 Outback does NOT have an EGR, the 04 Forester did, at least the 04 5speed I owned did, So you may have to get a plug for the port on the rear of the one head. 

That's the easy way to go. It's when you NEED and EGR port and your doner engine doesn't have one that causes a problem.

Cool. Thanks everyone