Volvo penta md6a impossible to start

Hi everybody I'm filippo from Italy and I'm so desperate that I had to join an English forum to try to solve my problem
i've bought a boat, one year older than me (40 yo) as the Diesel engine on board, the engine has been stopped during the last 5 years, but 6 years ago has been fully reconditioned and it worked good.
Now... It's impossible to switch it on, I've tried and checked everything:
1- two brand new batteries (one 100Ah and one 74Ah)
2- injectors cleaned and tested (they spry with a very good pattern)
3- oil filter and oil changed
4- valves checked and registered (cold engine I know but I would do it again with hot engine)
5- I've tried with a starter spry, nothing
6- I've tried blowing hot air in the air intake, nothing
7- I've checked the exhaust collector, it's clean!
8- I've closed the water cooling supplying line to avoid water in the cylinders
9- I've checked if the cylinders are dry, yes they are
10- the compression seems good, the starter can barely make a complete turn when the valve lever is down, I should make a compression test

what else should I have to try? An exorcist? I don't know what to do
does anybody have some suggestion?
Thank you in advance

On an engine this old you may have a dynostart, which is when the dynamo is fed battery power to spin the motor over. However these are nowhere near as powerful as a proper starter motor. Or maybe you have a regular starter that is weak.
Try moving the compression lever to the position where it opens the valves and there is no compression, then run the starter until the engine is spinning as fast as possible then flip the lever back, it should fire up, it may need some startgas.