Not sure what you mean by core module, but at risk of too much self-promotion, my Solar Service Module on OrbitHangar fits, and should be able to be made to fit in a Shuttlefleet payload bay if it isn't already. (it's been a while since I looked at it )
Thorton's Mir has the Mir Core Module, which should fit. But you'll have to add a attachment point to it's .ini file before you can use it on the shuttle.