Hi Damian, I am no expert on the lister L but I am happy to share what little I do know. The fuel system on thee incorporates a lift pump mechanism that lifts fuel from the tank to the carburettor. They do not have what we would recognise as a float chamber so I suspect that gravity feeding from a raised tank could flood the cylinder. I would recommend fabricating a larger tank and fitting it in the original position using the original lines. Alternatively, it might be possible to fit a float chamber to the side of the original tank and then gravity feed that from a larger raised tank.
If I remember rightly, these had a brass valve on the side of the cylinder head, this was a decompression valve allowing one to crank the engine without busting a gut. Older engines that have lost some compression probably don`t require it`s use. If you are going to use it don`t smoke as it pumps a nasty mixture of air and gasoline into your engine shed.
Bob