For those are considering a second powered unit with a Williams engine, I have done the upgrade kit on an F-7 A unit using Bachmann's upgrade kit. Two problems that I encountered are; first was to drill the hole a bit larger for the pilot or step frame to mate up to the newer engine frame, second the new motorized unit ran faster then the old one did, so I swapped out an older frame & motorized unit to make the two powered units almost equal in speed.
Lee Fritz