Hello,
the matching steam whistle for the small machine is just offered on ebay Germany.
https://www.ebay.de/itm/144003031763?hash=item21873fe2d3:g:8WUAAOSwFN9fgEieThe price is still within normal limits. The whistle should fit. You should measure the internal thread in the boiler of your machine as a precaution.
Information from the seller:
Pipe for Doll steam engine
Thread 5,1 mm
Height 4 cm
Shipping to Australia and USA is currently by sea
I have bought from this seller more often. He is absolutely trustworthy and correct.