Thanks to each of Bughunter and RAGII for their responses
Using Bughunter's Fokker drawing it appears the overall length of a Mercedes engine in 1:1 is c. 1338mm ( I'm cutting off about 200 mm in 1:1 for the thickness of the prop and its boss )
That measurement converts (using this marvelous tool -
http://www.scalemodelersworld.com/online-scale-converter-tool.html ) to c. 27.875 mm in 1/48. Nearly all kit supplied engines are quite a lot smaller than that.
RAGII's right of course, most accurately true-to-scale engines won't fit inside the thickened walls of a plastic kit's fuselage or cowling ...
but that fact doesn't justify fitting an exposed engine that's significantly undersized just because it was supplied with the kit
If you're doing an aircraft with a fully cowled engine then, of course, the above is taking AMS to a completely silly degree - But if you do aircraft which were built or often flown with fully or substantially exposed engines, don't you want something that is as close to scale as possible ?
In part answer to my own question about the Hiero 200 hp engine -
There's no after-market option I can identify - One of our Austrian modeller friends (Peter Plattner) was going to produce a 185 and 200 Hiero a few years back, but, to the best of my knowledge, that never happened.
The only kit I know of which supplies this engine is the Special Hobby or Blue Max Phonix DI/ DII
Regards to all and thanks again to Bughunter and RAGII
David