HelloBelfrybat,
Everyone is right in saying that a Singer 15 (any 15) is too big for a cabinet for a 99. The length and breadth of the base of a 15 are larger and the hinges are further apart. The 15 will not fit in a cabinet made for a 99.
The reason that there is a gap to the right of the 99 in the cabinet is that this is the space that would accommodate the treadle belt - but there is no belt because it has a motor.