Welcome to the Forum.
As suggested take the circuit board out and gently clean it up using the least damaging solvents possible. Try it on an area that wont matter if damage occurs. Clean dry soft brush first even using compressed air to aid the cleaning. Electronic stores sell aerosol cans of solvents suitable for cleaning circuit boards but still be careful using them.
The last thing to do is give up on these well made old generators nothing new stand up against them on build and quality. I know very little or less on electronics but I have been coached into attempting a fix on a circuit board on a MIG welder. I now, almost know how to identify a transistor, capacitor and diode by reading the board. I still have a long way to go to work out what the values are or mean but with time and patience I am going forward.
Freeing up the brushes in the brush holders is very important and I mean 'clean' not just so they can move in and out. Also the contact surface on the armature must also be cleaned up using some fine wet n dry paper and I dont use any water when I have done these in the past. New brushes should be readily available and if not easy to get a hold of I have bought some that were a little bit bigger than the originals and filed them down to size. Like anything the cleaner every thing is the better it works and also runs cooler.
Look forward to seeing more pics of the generator and the board.

If the threads are in good condition no sealant is required. I have been in the situation where a sealant had to be used so I used a product calle STAG Joint paste. I applied it to the thread and tightened the joint and left it 24 hours to sure and that worked fo me.
With the banjo fittings the new soft fibre washers are what makes the seal not the thread so again with the washers no sealant required.
Some fuel lines use an olive fitting which compresses onto the fuel line when you tighten it up. If the fuel line is sitting square in the hole and the seats for the olive to sit into are clean then this should seal fairly well with a slight chance of a leak past the thread.
That was very good advise to NOT use the PTF tape it is not recomended on fuel systems.
Other products are out there and from memory the Loctite range have a very good sealant which is like a gel and use it as directed and it will work.
Is there any reason why thi engine has 2 in line fuel filters? I believe that one is enough.

Now that diagram is a huge help. That is not the first time that so called professionals have stuffed up. They are not always helpful with these older engines and possibly never stripped one. I had an apprentice stuff up a simple tune up on a chain saw once. Then before leaving I had a quick inspection of it as I knew it still was not right and found a few mistakes which he denied. I gave him a piece of my mind and it was not nice for him which bought the boss out of his office and I showed him what his unsupervised apprantice had done and told him to wipe his @rse with the bill as I wont be paid.
Sorry, back to your little problem. With the metal washer it might fit onto the bolt that holds the bol to the body and the "O" ring fit between the washer and body.
For the missing spring. Fit the filter with the large hole facing up but no "O" ring ontop of it and measure from the top of the bowl to the top of the filter. This is or should be an ideal length spring you need to buy. It does not need to be very strong just enough to hold some pressure against the filter body. We have a hardware chain here that has a section of both extension and compression springs of various gauges and lengths. If you find a light spring and is too loong an angle grinder sorts that out quickly. I have just done something similar recently with a throttle return spring in a Mk2 Villiers carby. Have a close look up inside the body where the filter with the "O" ring should fit to mke sure there is not a step that will require the new spring to be shorter again.
It may also be an idea to check both of your filters to make sure they are both complete. I am not sure why you need 2 of these filter set ups so maybe one is enough for your new to you, narrow boat. Would like to see a pic of it too one day.

Do you know exactly where the fuel is leaking from. If it is leaking from the large "O" ring that sits between the housing and the bowl then it is either cracked, broken or not seated properly. If it is leaking from the fuel lines in or out then tell us what type of fittings are there. If there are olives then maybe they need to be replaced. If there is a flare joint then look for any cracks in the flare.
Finally the air bleed screws if the "O" rings if fitted are damaged or simply they are not screwed up firm enough but do not over tighten. Can't remember what the filter itself looks like but I didn't think there was a top and bottom to them. Would need a pic of what you have and of up inside where it sits to confirm.

Great news and outcome. Now go out there and make it run for hours and while that is happening find something to attach to it so it can be under load when running, they love it.

Love seeing steam but out here in Oz you have to have the boilers certified along with a certificate to operate a steam powered anything. We sometimes go to places just to watch these silent monsters tick over.

Hi Mike,
Boy haven't I been left in the dark with improvements. I have not heard of these type of senders but I do live almost at the back of a cave here in Oz.