A tropical stout can be a good substitution, many ways to add sweetness and mouthfeel.
Dave’s post (copied below) is the answer to your chocolate cake stout quandary
I don’t understand why we’re not all talking about maltodextrin. It definitely works. It’s not very sweet, but that’s fine, then just use a lot of it. Perhaps 1 to 1.5 lb per 5 gallons. Yes it will add body. But it will add some sweetness as well. Almost the same as lactose, but it’s not lactose. If you want to offset the body portion of it, then mash really low for a long time and/or use a highly attenuative yeast to balance the body out. Plenty of knobs to turn here besides messing around with enzymes which is almost certain to ruin a beer.
Just to add my 0.02 to this. I use maltodextrin in my imperial stout. It adds the body I am looking for and is not very sweet as Dave said. I only used 0.5 lb. in a five gallon batch and it was good. I don ’t think I would want to go higher in my specific beer, since it is an imperial stout, but you can experiment.
I would use maltodextrin as well