As a welder I can tell you that the crack can be welded with either TIG or MIG. Brian is right that if it extends through the seal area it will require a bit of creative welding and grinding.

Get the local Yellow Pages and call a few welding shops in your area. Ask them if they have aluminum welding facilities and if so what kind. Take the cover to the shop and ask for an estimate. Look over the shop and see how clean it is. Ask them how they will keep the cover from warping. A cover with a flat side like yours can easily be bolted or clamped to a flat work table. Ask to see a sample of their work. Ask which type of welding they recommend for the cover. Personally I like TIG because you have more control. Try 4 or 5 shops if you can. I bet their prices will vary by 100%. If the crack goes into the seal area then there will be some hand grinding which will run the cost up.

Before you get it welded find out how much for a used cover. The welding job will easily cost you $75-100 and if you can get a used one for $100-150 it might be worth the effort.