Engine cover...I wouldn't put in an oven, though it would probably be ok if you protected it. However, engine paint dries pretty quickly. For the parts you are planning on painting, the need for baking doesn't seem necessary IMO.
Valve covers are easy (I've done two sets....twice
) They dry to the touch enough to reinstall them within 40 minutes of the last clear coat. After a 45 minute drive mine were pretty much cured just fine. The directions on the spray can usually says drive for one hour to let cure.
Top? You mean the upper intake?