As an alternative, you can also use two small 12v batteries, wired in parallel. Wiring in parallel doubles the cranking amps and keeps the voltage the same: the normal 6v batteries are wired in series, which gives the same cranking amps as one battery, but doubles the voltage.
I found two small 12v batteries, which combine to give 780 cranking amps: more than enough for most engines! Certainly useful for V8s, but the 1800 engine can be a bit of a pig to turn over, too.