Do you own a later model electric start or a kick-start? For the former, a lot of folks here have recommended an Odyssey make (PC545), but if you are running an earlier machine, you can probably get by with a mere "scooter" battery, something like a 7-12 AH, which will free up some good space and cut off some weight as well.
This is what I use in my 74 Roadster. Don't know if your battery box is the same size. It is smaller than my stock battery, so gives me some extra room. Had to change the terminals. Scroll down to: LC-P127R2P(a) which is 12v 7.2AH.
I have been running one for the past 3 years running a Boyer ignition.