The trad Plus 8 had a bulkhead mounted battery, unlike others of the era. The space (height) available was limited so a compromise was sought in a smaller battery, the inevitable reduction in capacity being offset by using a premium one with high cold cranking amps (CCA). The most common replacement seems to be an Odessy PC925 which has a CCA of 325 amps to offset the minimal 28 amp hour capacity. The consequence is that perhaps they don't last as long as the rear mounted nominal 55 amp hour capacity batteries used in other trad Mogs. Some Plus 8 owners have re-engineered the main battery leads to accept a larger gel rear mounted battery lying on its side, I assume because of space limitations.
Regarding your starting problems, have you checked the charging system
With a near fully charged battery and after a few minutes of running you should be seeing in the order of 14.4 volts at the battery terminals with engine at a fast idle.
