The clutch is either not working, or the motor is wired to spin backwards.
Did you use citizencycle's motor swap youtube video? I've never taken my G06 apart, but you must have done what he did, leave the freewheel on the motor and try to remove the screws uder it. He had room. You didn't. There are freewheel removal tools.
If the clutch is working right, I think your wheel will have resistance when you turn it backwards by hand, and spin easier going forward, If it spins equally easy in both directions, the clutch isn't working and I believe you made a mistake putting the new core in the old housing. Left out something that makes the clutch work..
This shouldn't happen if you put a bafang motor in place of another bafang motor, but some people have swapped motors and the manufacturers didn't use the same color codes on the wires, and it has happened that the new motor spins backward. On a geared motor, if it spins backwards, the clutch lets the wheel not turn. I think this is very unlikely if both motors are bafangs.