Is all good, sometimes we can read things and mis-interpret them, I'm guilty of that a lot.

Something that may help you out is to go to YouTube and type in what you want to do. You will be amazed at how many videos are out there on every topic you can think of. I am about to retire and only about 18 months ago I bought a cheap Chinesium metal lathe 2nd hand. I have then watched many videos and started to buy more tooling and even had to make my own thread chasing dial because my lather has a metric lead screw and I need to make BSF and BSW threads. Yes, I could stop the lathe and reverse it but this has no brake on it so damage would happen. My research paid off and now have a dial perfect for the job.
So have a look and see some options that might help you out with the tools you have.

Are you certain that the taper should be there? I haven't come across that design before. It has always been a parallel fit with either circlips or even a lead plug hit into place to keep it there. Even if the fit was a bit firm in the piston to help hold it.

I also use a short lngth of garden hose if the long screw driver isn't handy.

For water jacket covers I would use the thicker gasket paper along with Permatex #2 sealant. It does not set and can be broken carefully and the gasket can be re-used.

Glad you found a problem. The fracture can be brazed again and at a reasonable cost because of how quick it can be done. As for the pin holes depending on how bad they are as to which way to go. These too can have a large patch soldered or brazed over the area but to make that decision you will need to remove the paint with a light grind to be able to access the damage.
A tank repair kit might be able to do the job but again you will need to see how good or bad it is by removing the paint and I would clean the complete bottom area of the tank first because if it has pin holes in one place there is bound to be more.
Hard to offer best advise without pics but in the mean time I would search for a replacement tank which could work out cheaper than a repair. There are Dumpy parts suppliers in the UK and I think mainly Winget parts but a fuel tank is interchangeable with them or should be.
Have a good look inside the tank through the filler cap and see how it looks inside.
Some very basic methods of cleaning up inside tanks can be the use of blue gravel and shake it up for a long time to loosen the rust.
Electrolysis works very well for cleaning metal parts up. Mix water and soda ash. Place the tank in a large tub so that it is submerged then an anode hanging inside the tank and the other lead to the tank. The anode must be insulated and not touch the tank itself just hang freely inside then connect a battery charger to it.
The use of Hydrochloric acid works too but needs to be neutralized extremely well by the use of a lot of clean water.

Gday Jacinta and welcome to the forum.
Not sure iff you can see the fuel line connection cleaarly while it is in place but if you canwipe it as dry as possible and with a good light see if you can pin point where the leak is comming from. Some ideas are;
Hole in fuel tank.
Fracture of the solder/brazing of the connection where it is attached to the tank.
Your fuel line may need to be replaced due to a flare or olive failure which is how most fuel lines seal. (I am going through this now on one of the tractors).

To fix this leak will depend on where it is comming from. If it is a hole than a patch might be able to be soldered or brazed on same for a leak around the outlet. Here in Australia there are fuel tank repair kits available and these are a simple 3 step proceedure but pricey. A kit to do a large fuel tank say from a car is around AU$150.00. If it is the fuel line and you have no way of making another one take it to your local hydraulics shop and they will make you a new one that will bolt straight on.

I have an SR2 engine with the fuel tank that holds around about 1 gallon so it can easily be unbolted, lifted ot and emptied into a fuel tin using a funnel. If you have a larger tank then either a syphon to get as much out as possible or disconnect the fuel line somewhere so that you can then push a flexible plastic hose onto it and let it drain into a fuel can.

It is not a common problem but if it has gone rusty inside maybe finding a replacement tank is best and you should be able to adapt any kind of tank to do the job so long as you can mount it above the injector pump to be a gravity feed it will work. I used an old plastic lawn mower tank for a few years on our old 8hp CD Lister until I found an original tank that held about 2 gallons.

Good luck with finding the problem and keep us informed as to what you found and what you can do to fix it. Others may chime in with better ideas and help.

Welcome to the Forum,
As stated value will depend on its condition as to if the engine has seized and what condition the engine is under those covers. At the end of the day they are all worth saving and some if only for spares parts. If you don't want to keep it try advertising on your favourite site if no interests and you still wish to see it go to a good home look up a Vintage tractor and engine club near you and ask if they have a young new member who would like to have a project engine to do up as a gift from you to them. This keeps an interest in this hobby and will give a young bloke a great head start to a collection and interest in saving these gems.
I have donated a few small engines and mowers over the years and the young guys who ended up with them are still in the hobby and always come and say hello and talk about the next project/s they have found. But at least don't let it go to scrap.

There should be somewhere an engine ID tag attached to one of the covers somewhere. This will tell you the model and serial number for easy identification. The 'Dursley' part is the name of the City where the Lister factory was and were made.