Paul
Most likely it is.
To confirm your suspicion, measure the DC voltage across the battery WHILE THE ENGINE IS CRANKING. If it drops below 10 volts, the plates inside the battery have deteriorated from sulphation and are acting just like corroded wiring terminals in the starter circuit.
Believing a newly rebuilt SINGLE CYCLINDER engine might need 50% to 100% more battery capacity to break in is not true. If it were, we would never find a battery big enough to be able to start a rebuilt 8 cyclinder engine !
Going with a larger CCA battery in an environment subject to significant vibration may not be wise either. To allow a battery to produce larger CCA, manufactuers put in a larger number of MUCH THINNER lead plates to gain additional surface area. This configuration allows for a larger chemical reaction to take place and hence larger current flow. Thinner, weaker plates produce more current but are more susceptible to damage from vibration.
I have said this many times in the past - and it's probably worth repeating - anyone who has a number of vehicles or lawn care products containing lead acid batteries should own an inexpensive battery load tester. HF has them - so does SCHUMACHER. A 30 second test with one of these can save you $$$ worth of unecessary parts replacements and hours of troubleshooting. Some folks who know this to be true may even admit to saving DAYS of troubleshooting.
p.s Failing all the above, you might also want to check for the 30 - 40 thou lift from the ACR mechanism during slow speed cranking.