[Missed Approach]

Hi Fizeus- To avoid confusion, Alodine 1201 (now known as Bonderite MC-R 1201) is for aluminum alloys only, not Mg Alloys. Your fan shroud was an AZ31/AZ91 Mg Alloy, so (as you've already discovered) the Henkel Bonderite M-CR Magnesium Aero Conversion Coating 598970 is still the best option for an actual conversion coating. Unfortunately, prices have certainly climbed! A lower-priced alternative is certainly a spray like Gibbs Brand or Boeshield T-9. Sadly, these will not give the appearance of the factory conversion coating. However, if you review the forum, you will find that many have been pleased with the protection offered by either of these two sprays.

Hope this helps!