The "must have" is not the same as fitted by the factory, I cant tell you off hand what the original size is, but |I can tell you that on my 750 Egli I always used a tiny battery, evreyone said would be useless, it came from a Honda 400/4 and was about one sixth of the size and weight, it still started the bike OK and never gave me a single problem.,so you can go a lot smaller with no ill effects.
CLEM