To avoid all the guessing around and lotīs of time do a compression test.
This will narrow down the posibilities to what might be the cause of the problem.
I agree with Pierre....there is still a long way to the death sentence
Pistons can be changed, cylinders can be honed for to bigger size and Yanmar
supplies piston rings for over size.
As to starting a diesel
engine U need
3. Enough Power from Ur starter engine/crank amps Batterie
4. INJECTERS...they can be pressure checked and recovered with ultrasound treatment
5. Very important !!!PROPERLY FUNCTIONING GLOW PLUGS...to check take out...they should be dry....if wet / covered with oil
.... means Ur engine is not new anymore but still a long way to go to the graveyard. Usually a new set of glow plugs and U are all good....when have U changed the last time if ever ?
Checking the Glow plugs would be my first test
2.Injectors DO THEY get fuel from the pump...unplug fuel lines from pump at the injectors > start engine to give a few turns and check for proper fuel supply
Have U performed those tests ....?
After that the compression test defenitly will be Ur best choice and next step of diagnosis
BEFORE U TAKE EXPENSIVE DECISIONS