How to replace lead acid batteries to lithium on a GP500 mobility scooter?

Hi. I have a Green Power GP500 (red) mobility scooter and want to replace the faded lead acid batteries, MYSELF, with its' equivalent (or higher, if this is acceptable to the engineering/electrics of the scooter) of lithium batteries. Am not a mechanic but with the correct and detailed steps given, i can install the lithium batteries myself. The cost, by Green power, is almost 700£ for the lithium batteries, plus to remove the lead acid and rewire to accomodate the new.

So, any knowledgeable help would be greatly appreciated!