The alternator failed this week on my Atlas engine and I had to find a replacement very quickly. I'm sure that the good folks on this forum already know a source for these items but just in case anybody is wondering where to find something suitable, I sourced a Guzzi/ Ducati unit from
www.electrex.co.uk The Electrex G92 rotor is exactly the same dimension as the Atlas unit , the only difference being that it's two phase instead of three [just leave one of the reg/rec connections off] It wasn't cheap at £150 but I didn't have time to shop around. The quality is excellent.