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If anyone has spares and can measure the sizes, they might be able to be sourced on Ebay/AliExpress etc - probably at a small bulk cost instead of Tamiya 'spares prices'
I recommend you just use M3 standoffs, and install a nut on the backside (of the gearbox); or, in the alternative, use M4 standoffs and drill out the hull holes and obtain suitable M4 screws.
Probably not the first choices, but...it would work.