Ferrari 412 / Turning Engine Over by Hand

You can access the crank bolt from underneath easily enough with a thin walled 36mm socket, I had one machined down.

 

If you can find one of these sockets, it will fit without machining.
https://www.ferrarichat.com/forum/posts/146627533/

 

Fill it with water ? You'll end up with water in the sump.

 

If you can access the underside of the car, you can turn the flywheel using a screwdriver as a lever against the teeth of the flywheel. There is a small plate (the size of two credit cards) that you have to remove in order to access the flywheel.

If I were in your shoes, I would try first to use a magnet in order to recover the part in the cylinder. You can either buy a tool (looks like a flexible screw driver with a magnet), or you can wrap a neodyme magnet around a coper wire and "fish" your parts in the cylinder chamber. That's how I garbed the acorn nuts that had fallen in my cylinder head...

If you still want to flood the cylinder, Instead of water, maybe you should use gas-oil?

 

Some plastic won't float in water - I'd confirm that first with another piece of plastic in a glass of water.
Also a flexible claw pickup tool might do the trick -

Image Unavailable, Please Login

 

I have retrieved many non magnetic pieces from cylinders. One is a vacuum. The nozzle needs to be a close fit to the plug hole and at least one valve needs to be open to create turbulence and air movement or the piece will just sit where it is. A high capacity vacuum is needed. Another choice is to close both valves and blow a high volume of high pressure air in there. It often eventually gets out the plug hole. For this you need a small long nozzle to reach inside the cylinder.