@TURIN,
My company keeps entire engines (QEC an' all) built up to minimise AOG time during engine changes.
@Davor,
We also issue FORM 1s to removed components, with a statement in Block 12 detailing that the component may only be fitted to the same operators A/C that it was removed from (regardless of the MRO removing/fitting the component).
A representative from the operator can certify removed components as serviceable.