Disconnect coil wire, pull all the spark plugs, and put 1 oz. of motor oil in every cylinder.
Take a can of WD40 (or equiv.) and spray it around the cylinder walls as best as you can. Let it sit and soak for 10-15 minutes.
Pull the belt, and use a strap or chain wrench(with a rag for pulley protection) to turn the motor perhaps 1/4 turn. This will gently break the rings free of the walls if they have rusted to them. Spray oil again, then another quarter turn. Repeat oil spraying until full rotation has been made, then turn it 5-6 more rotations by hand to get the walls fully coated.
With the spark plugs still out, turn the motor with the starter until full oil pressure is reached.
A trusted old mechanic once told me that he periodically sprays fogging oil (normally used prior to storage) into the intake for a few minutes after first startup of a motor that has been in storage.
Sounds like a good idea...
The fuel may have gummed up, so you may need to flush the fuel system as well.
Best of luck with your new project.
1986 300E 5-Speed 240k mi.