I would check out the main live (+) cable connection on the starter motor if you haven't already and check the bike's charging rate first. You should be getting between 13.5 and 15.5 volts at the battery terminals with the engine running and lights off.
If you're looking for a Yuasa bigger amp hour battery then you need one of these (see link below). That's the one for my bike (year 1999, standard battery dimensions:- 150mm length X 87mm depth X 130mm high). The bigger amp hour battery in this link is 15mm higher, and as you mention, you will need to remove the battery box lid:-
http://www.ebay.co.uk/itm/151695234759? ... EBIDX%3AIT
Watch out for Chinese copies if you buy elsewhere.This company ( countybatteryservicesltd ) has confirmed to me they only sell genuine Yuasa batteries, no I'm not on commission
Chris.