I installed a shoari 18ah battery last June and it works great so far even with the old Boyer EI.
I do keep it on a battery tender just to keep the voltage up to max. It will crank the bike numerous times if it won't start right away, which hasn't been an issue since I replaced the plugs, wires and coils with new. Also adding new Amal premiers made a huge difference in starting. Most times it's one kick or one blip of the starter. The previous owner had already installed heavier battery cables which I think also help get the juice to the starter.