Drop it in a container of fresh (green ) anti-freeze / coolant...(coffee can maybe? )
There have been numerous threads about cleaning engines in a crockpot full 'o anti-freeze...

You don't nessessarily have to heat it up though. If you're willing to let it soak a week or so, and the a-f / coolant has rust inhibitors, etc. that will loosen rusted bearings and varnish...

It really does work better though if you heat it...