I followed the cooking instructions from a Julia Child's book for making a plum pudding including "Use a special pan made for this purpose....with a very tight lid," "Let the pudding cool in its container. Store it in a cool wine cellar, or in the refrigerator." I bought a special plum pudding mold and made my pudding. I cooled and refrigerator-stored my pudding for a month when my built-in fridge's compressor died. While in shop being repaired, I stored by pudding in a bucket with ice packs. Due to condensation, I found rust marks on the plastic bucket from the outside bottom of the mold. I began to worry about the pudding, with acid ingredients, being left in the metal mold. I unmolded the pudding and found a small amount of rust spots on the inside of the mold. The inside of the lid was terribly rusted but hadn't touched the food. Since the metal content isn't known (although with rust, does it have to be an iron content?), could some poisonous toxins have leached out too? Are cooking containers made safely now to avoid tin-can type deaths as in the past? Can just the outside layer of the pudding be cut off and the rest will be safe?
If it's only rust, you should be fine. Rust is oxidized iron and harmless. Check for other discolorations or changes in surface texture, that means something else is happening there If in doubt, ditch it.
Here's what I'd do: I'd cut off (rather generously) the rusted parts, sample a bit of pudding from the inside and decide if the recipe is worth another try in a new mold. Then, get yourself a new mold. This time, make sure it's appropriate for your intended use: Some puddings are cooked in the mold, then served right away (so no danger of excessive rusting), some , like your plum pudding, need longer time to "ripen". Maybe you coud get a porcelain mold?