Generally speaking anodizing cast parts will yield less that desirable results, though there are exceptions. In the case of old motorcycle engine parts I'd say wave buh-by. Cast aluminum tends to be somewhat porous and absorbs a bit of oil so even if it doesn't burn up in the acid bath you're still likely to fail the sealing process.
A safe rule is that anything milled from a chunk of 2000, 6000 and 7000 series aluminum will work for anodize.
The main reason for failing the process however is what the base metal is 'cut' with. Most cast aluminum is alloyed with tin and zinc to make it flow better during the casting process. These metals do not react well to acids and even worse when current is passed thru them to open the substrate. Anodizing is in essence, a form of controlled corrosion wherein electric current is flowed thru a sulfuric acid bath into which the aluminum part is then dipped for a certain amount of time. During this time the outer 'skin' of the piece oxidizes, usually to a depth of 2-5 thousandths of an inch. The oxidization takes the form of sub-microscopic scales that stand up on the surface. The piece is then often dipped into colored dye to add the nifty colors everyone loves. The freshly dyed part is then boiled in water to 'seal' the dye in by forcing the oxide scales to lay flat again, thusly sealing the dye into the surface.
Pretty colors are nice, however the real purpose is to halt further oxidization. The scale layer is extremely hard and stops oxygen from reaching the raw aluminum underneath.
There are some anodizing processes specific to cast aluminum that can be done, however the end result is an icky grey/brown/coal colored usually rough finish. Good for highly corrosive environments but not so much for looking pretty.
