It's probably the remains of the rear engine buffer - most are a single piece of hard rubber, but some are a two-part item, with the rubber bonded into a steel backing plate. It's been a long while since I've seen one, but back in the dim and distant past, I had to remove one from an engine where the rubber had un-bonded itself from the steel and disappeared. I think I had to drill a series of holes through the steel backing to weaken it, and then prised the remains out with a screwdriver - I remember it was a pig of a job, though. If the engine is in pieces, it might be possible to heat the engine case enough to persuade the steel piece to drop out due to the greater expansion of the aluminium, but drilling (and maybe grinding the steel plate with a Dremel?) is possibly the best option. Don't be too enthusiastic with the drill, and stop when the turnings change from steel to aluminium.....
