Hi JDE
Zero experience here.
I am guessing that you have no experience in diesel engines.
You probably have a Yanmar 1GM10.
These engines are tough, simple and very reliable when running properly.
First thing is a manual. Read it! It will fill you in on the essential maintanance to keep your little donk purring.
Yanmar 1GM Marine Diesel Engine Service Repair Manual - Download as a PDF or view online for free
www.slideshare.net
Check and see that the piston isn't siezed in the barrel for a start. The is a decompression lever (refer to manual) and a cranck handle. If it turns over you are in business. If not, start to count your dollars.
Next tools. luckily you don't need any special tools.
Metric socket set (up to 23 mm or so)
Metric open end ring spanner set. Up to 20 mm
Metric open end spanner set. Up to 20 mm.
Metric allen key set.
Plus and minus screwdrivers.
Hammer and drift punch. and a few other things but all in all not a lot.
Now starting an engine that has been sitting for three years.
If you just connect to a fully charged battery and hit the starter, you might start it but you will also create some problems for yourself.
So take a step back and prepare.
Take it for granted that the fuel in the tank, lines and filters is contaminated (with water through condensation, rust particles, and a growth that thrives in diesel fuel under the right conditions).
This means you will need to drain all of the fuel. Work out how much is in there and prepare containers, funnels, gloves, rags and whatever else you will need. Drain the whole system. If there is an inspection cover on the tank, remove it and clean inside the tank with rags. This is not just being done to get the engine started, but to ensure that you have no fuel problems in the future ( out of sight of land). This a messy , smelly job at this stage but will only need to be done once if done properly.
Replace fuel filters, add new fuel and bleed the system. This is something that you MUST learn, so you might as well get the knowledge early. Luckily you have a single cylinder engine so it not so difficult. but you must do it correctly.
Ok fuel system done and familiarized.
The engine is cooled by sea water drawn in through a sea cock. Then to a filter and the pump.
The pump has a rubber impeller inside that could have deteriorated ( see manual,) check it.
Again this is a must to know and to be able to do.
Cooling Ok.
Diesel engines don't have spark pugs, The fuel is detonated by the heat generated by the compressed fuel and air. This means the engine has very high compression. To start it needs to turn over quite fast to generate that heat.
So as mentioned by Capta, get your self a new battery and have it fully charged.
That's it. Well I hope so anyway.
gary
PS while I was writing this you dug up a manual. Good work!!