Hi Mate,
The Motorplate for the Tamiya Bigwig is the same as the one thats on the Boomerang.
The Boomerang has recently been Re-released(I've just bought one
), so you can now get the Motorplate quite easily.
Almost all of the Gearbox Parts are the same on the Bigwig & Boomerang, but Tamiya has Changed many of the Part Numbers on their Re-released Kits, so its made it alittle confusing for People, who want replacement Bits for their original Models.
The NEW Part Number for the Motorplate is 4305125, which Costs around £1.25 from your Local Tamiya Dealer.........at that Price, its hardly worth trying to make one! Eventhough it looks like that its only made of thin Cardboard.