As to, same size due to battery box
size, shim as necessary, when necessary, where necessary.
A heavy drain on the batteries, when attempting to start would lead one to examine the starter circuit for a short.
However, a lot have suffered from water
intrusion into the engine
cylinders, via exhaust elbow
Although, not perfect, one can draw additional power
from the house bank, to jump start the motor
Then one must affect repairs
, as is necessary, provided you haven't sent a rod tearing through your engine block due to the aforementioned water
Then again, there are the sails