Thanks everyone! I must admit it would be a lot easier without the guesswork. I just can't find pics of these early Argus 6 cylinder engines with the flat underside to the crankcase, so I'm basing most of it on pics of the 4 cylinder engines which appear to have been very similar in layout.
I got a few more hours in this afternoon, most of it trying to work out what needs to go where, and what size stock I should use. I did get the magnetos done, using HO scale 4 x 6 (1.09 x 1.68mm) rod and fitted them to one of the blocks, along with the PE gear that I have from the Miniworld Scarff ring, so that I could then gauge the sizes needed for the other gears. I think I'm going to use left over PE Vickers gun ends from an Airwaves set I have (Airwaves have to be useful for something surely?) for the other large gear, and just use plastic rod for the mag drives.
Here's how it looks with those bits on...

Ready for some paint I think, then I can start to think about adding the cylinders........
Thanks for looking in!
Ian