Cleaning old Engines

I have bought a couple old engines off of ebay, they looked great in the pic but when I got them the side that sits in the box has all kinds of stuff on it. Its looks like old foam. I tried to wipe it off but it not going anywhere. Any suggestions on what to use to remove the foam without hurting the plastic?

Thanks

Schom

Heres my method…Remove the shell from the drive and proceed to wash it in the sink using a SOFT toothbrush or paint brush let air dry while cleaning and relubing the drive…

To add to Larry’s fine input, use Dawn detergent, do not aggressively spray.