I would use some spirits cleaner suitable for electronics , there are several types and sources, Amazon for one.

 

That and a toothbrush or an industrial one and brush lightly and wipe clean with a lint free towel until all the contamination stain is as removed as possible. Several "washes" will be needed to remove all the loosened residue muck. Unplug and clean all board connectors. The wiping action of plugging and unplugging a few times helps. We used a No. 2 pencil eraser on board edge plated connections to burnish any surface oxidation off back when. And good luck! Hopefully nothing shorted out due to contamination. I also have some various sized lint free swabs that I've bought and in a pinch a Q-Tip swab but it tends to leave strands of cotton behind that have to be cleaned off.