50Ah of lithium will definitely be lighter, but will only be 50Ah, not the 300Ah that you have now.
You could achieve a 66% weight reduction (about 60kg) simply by removing 2 of those existing batteries and still have double the storage of the lithium that you propose.
Cheers,
Peter
Thanks mate, sold the lot from Gumtree before I saw your reply. I am about to advertise the charger here which is a top quality EPS1225. Just letting you know.