If you mean the beam engine with the pair of gears that double the speed of the fly, it was beatifully illustrated in Encyclopaedia Britannica (3rd Edition 1797) Vol xvii illustrating John Robison’s articles Steam and Steam Engine.
I glean that from the caption of a small copy of the engraving in “”Watts Perfect Engine” by Ben Marsden (icon Books, 2002, ISBN 1 84046 546 8). This is an excellent biography of Watt.
A different engraving, apparently amended and extended to include the boiler, from Farey’s Steam Engine appears in “Power from Steam” by Richard L Hills (Cambridge, 1989, ISBN 0 521 45834 X) which claims to be a history of the stationary steam engine but really covers the evolution of pumping engines into mill engines.
Interestingly both books use the respective engravings of Boulton and Watt’s rotative engine on their covers.
Neil