if you're handy with a soldering iron, the trick is to use the smaller late 748/9x6 battery and re-locate the retaining clamp.
Unfortunately the +ve and -ve terminals end up ar8e about face so all the wiring has to be extended.......
As I don't have to worry about trivia like lights I have got a spare loom and hacked it about.
It will just be a pain in the neck when/if it ever goes back to being a roadbike as I will have to change out the whole loom
Plan B said get a race battery of a suitable amp rating with the terminals the right way round, but that only occurred afer I started cutting.......