Will the head come off my EJ251 while the engine is in the car? it in a 2000 Subaru Outback.

wwwRockauto.com should have them for you.

This is common issue early sign head gaskets failure for EJ251 SOHC engine for external coolant leak. Second gen 96-99 with EJ25D is worst because when head gaskets failed, it's an internal leak that allows coolant and oil mix together in the combustion chamber to destroy engine.
You can drive it for at least 6 months or more as long as you keep top up coolant since the leak should be small at a slower rate leak. I drove mine for a while before change head gaskets. You're looking at `$1000-1200 job a dealership since you might as well change the timing belt, water pump, idlers while engine is pulled to replace head gaskets.

If it has one,it will be at the bottom,engine side,could be in a super tight spot to get to,you can remove hose at bottom of radiator to empty radiator,will be a lot quicker.

My first thought would have been a failed coil. Most 4 cylinder coils fire odd cylinders together, one positive and one negative voltage.
The crank sensor could be faulty too, but I would rule out the coil first.

Yes it will bolt in.

Take off valve cover an rotate top pulley for camshaft. Watch all your valves operate up and down. If they go through full motion up and down and you don't have any binding and everything looks straight as it goes through opening and closing while you are rotating shaft then you should be fine on the head.

thelight comes on, one or more diagnostic trouble codes (DTC) are stored in theengine control module.

you need to pull the timing covers to see if it has a broken timing belt and find out for sure if it is an interferance engine, could be a good deal if it just needs T-belt.

CODE PO420 CATALYST SYSTEM EFFICIENCY BELOW THRESHOLD BANK I * CODE PO325 KNOCK SENSOR NO.1 CIRCUIT MALFUNCTION BANK NO.1 OR SINGLE SENSOR

Thats not many miles for a Subaru. The only thing that really ever seems to cause problems with Subaru motors is the Head gaskets. But, with proper maintanance they last a long time. If caught at first sign of trouble repair is not alot of money. I have 3 Subaru's with over 200000 on them and I will take them against anything. Maintance is the key to repair problems. Subaru parts are not expensive and easily available. It will probobly be another story of buying a Subaru and never buying another car other than that. Good luck and have a great day.