Taking off and replacing the old freewheel off a golden motor pie kit can  be a little bit tricky.  Here are some tips that might help you. 

1) Purchase  the "Park Tool FR-1.2"  or other similiar freewheel removal tool.

It has a .403" thru hole. The axle is .545". In order to remove the freewheel, the tool must be drilled out to more than .545".

2) Be sure to install the spacer in between the motor and the freewheel when installing a freewheel   This will keep your freewheel from locking up.