Yeah i had that problem too when we took out engine. It could be caused by a lot of things. Haynes repair manual says this...
Engine rotates but wont start:
Fuel Tank Empty - Maybe u forgot? LOL
Battery Discharged Engine rotates too slow - If u have a volt meter id check that second.
Battery Terminal Connections messeed up - just use a wirebrush if u have to clean em and put em back on, make sure there tight, and cant be moved by hand after u check battery.
Leaking fuel Injectors, faulty fuel pump, presure regulator etc. (i realy doubt this)
Ignition componets damp or damaged. < when we put car togeather we used Electrical connection GOO it realy helps, apply it wherever u conenct stuff, keeps the moisture out
Bad sparkplug - could be but u should have at least partial firing, i would check if there IS spark at all
Brocken or disconnnected lose wiring < i cant stress that enuff, thats the killer, make sure u have it pluged in
Faulty coil or its disconnected - Could be, i put a bunch of Electronic goo all over the connections of mine and so far so good.
Since its all electronic, if ur not getting firing it might be the FUEL or air delivery too. Thats the problem i had, u can try to start car by just spraying a SAFE EFI or carb cleaner can, its preaty cheap and if the car coughs and tries to start it must be fuel. If it doesnt, might be air or electronics, which is a pain

Usualy electronic stuff will still let it fire a bit. If u want to check it u can plug in Computer at worst.
Proper name is Di-Electric Grease
Goodluck hope that helps