If you aply the degreaser on the outside of the engine I dont see how it could hurt it. But my experience with degreasers requires some elbo grease or they are a waste of money. I have an airblasting tool with a solvent pick up tube, I drop the tube in a can of solvent, after scraping the motor, and go to town. Im careful not to set myself on fire or blow up the property in the process.
my guess is you are wondering if it will destroy old gaskets if touches them. If my guess is correct than the answer is, try to keep the degreaser on the metal only. it wont destroy a gasket immediately but if there is a crack in it you could get degreaser inside the engine. this is not good if you are running the engine. if a full rebuild is in order, then it wont much mater so long as the engine internals are coated in an oil for storage. hope this helps.
It wont hurt it. Your issue will be getting the ignition components wet. AS far as the engine its fine.
I have had great luck with purple power. It seems very mild but does extreme duty on grease. I use it on my driveway too. I buy mine at auto zone and not sure if it's sold other places.
m honestly I have used simple green and mean green, but neither compare to purple power. But, I would just avoid distributor, spray on motor and let sit for 15 min and rinse. Repeat as needed.