Regarding lithium-ion: I think these 32v 50Ah lithium packs are from Mitsubishi i-MIEV electric cars:
https://www.ebay.co.uk/itm/143124540922
Two of them would double your range, without needing the "range extender" generator, and weigh less than the present 55AH lead-acid. The packs have 8 cells, each cell weighs 1.7Kg, so 13.6Kg total. However, not cheap, and would need new charging/balancing circuitry.
https://www.ebay.co.uk/itm/143124540922
Two of them would double your range, without needing the "range extender" generator, and weigh less than the present 55AH lead-acid. The packs have 8 cells, each cell weighs 1.7Kg, so 13.6Kg total. However, not cheap, and would need new charging/balancing circuitry.