In as cast condition Magnesium has a skin on it that if left alone will last a fair time assuming it is not soaking in water or in a very moist environment. The easiest approach is to find someone to grit blast and chrome pickle the parts immediately after blasting. The chrome pickle is a chemical conversion basically a mixture of nitric acid and sodium dichromate dihydrate. Nasty stuff. You can buy this mix it and apply per MIL/AMS M 3171. It is the easiest way to prevent oxidation but should be painted soon after. There are some decent catalyzed paints you can use that are not perfect but will hold up to solvents which is a must for engines.
Eastwood makes a catalyzed primer and paints that do a decent job. http://www.eastwood.com/paints/2k-ae...ay-paints.html
There are no guarantees it will hold up but you might consider their clear over the polished but I have never seen that last.
Another option is finding a shop that can do the passivation for you. This shop is local to me and does excellent work but call to verify they can help you.
PS: it may not appear to be oxidizing but it starts within an hour or so after cleaning. You can't trust your eyes. Rule of thumb is some form of coating or conversion within an hour of cleaning. Also oils from your hands will clearly show up as fingerprints or black marks after a while. If this is allowed to sit for weeks it might show up after you coat it but years from now.
Magnesium is great metal but is temperamental. I would consider polishing any surface that has not been treated again prior to coating with anything. Also a good wash and dry at 120 degrees in a convection type oven is also wise.
Also others said that the gray powdery surface is from a conversion treatment. that is incorrect. Depending on hardness the magnesium will show up as a greenish gray to gold color not powdery. If it is gray powdery be careful this can be explosive if sparks are nearby.