Don't bother. Throw the whole thing out. It will taste nasty with minimal effects. Besides, just the idea of thinking that you might be poisoned is enough to ruin a trip. I said it, so if you didn't know, now you do. Contaminated cakes are really nothing to play with, especially if you can't identify the contaminant.
If you really want to do something with it, cut off as much contamination as you can, with one inch margins, treat it like cancer. Then spray the whole thing with H2O2, and then you can put it back into fruiting or bury it outside in some regular potting soil. No matter what, it is a gamble. I would only bother if it were a large 1qt+ cake, anything smaller isn't worth saving unless you have a bunch of them.