what size allen key for liftmaster gatestop and bolt?

[bomitie]

Hi

I need to change the hinge fixing on my liftmaster gates.  I got the parts but I need to transfer the gatestop from the old to the new but it is held with a nut that needs an allen key.  What size allen key do I need?  I should explain when I try to undo bolt the whole thing rotates. 

Also what size is the bolt that hold the hinge to the motor and where can I get them?

Thanks.