Hi Pablo,
That "small crack" appears to be an imperfection caused by either a small pocket of trapped air or a small piece of slag injected into the mould during the casting process and it would have been there prior to the machining process, which is why it still has the black paint visible inside it.
In my opinion it is only cosmetic and would have no noticeable affect on either the strength or the usability of the motor.
If it was mine, I would simply touch it in with some silver paint (or just scrape the black paint off) to make it less noticeable, this would eliminate any delays in the build along with the additional hassle of trying to obtain a replacement cover, having to swap the freewheel and bearing over and then refit and reseal the cover joint with silicone sealant etc..
Alan
