Battery voltage?

If the electrical system is originally a 6-volt system, switching to a 12-volt battery will only cause a lot of trouble. First of all, the generator will only develop around 7.0 volts, so no current will go into the battery to keep it charged. Until the new battery runs down, you'll burn out any light bulbs that are turned on, and possibly damage other electrical devices such as the voltage regulator.

By far the biggest suspect if you feel cranking speed is too slow is the wrong battery cables are being used. A certain amount of power is needed to spin the engine. Using Ohm's Law, Power equals volts times amps. A typical Ford or GM V-8 starter draws roughly 200 amps once it's up and running; more to get it started. 12 volts times 200 amps equals 2400 watts. To get 2400 watts from a 6-volt system, you need 400 amps, (6 volts times 400 amps equals 2400 watts).

To say that in a way that's easier to grasp, when you have half the voltage, (electrical pressure), you need twice the current, (flow), to get the same power. If you're using the battery cables you typically find in hardware and auto parts stores, those are for automotive 12-volt systems. You need 6-volt cables which are a lot larger in diameter. You may have to ask for them specifically, or look for cables used in heavy trucks or industrial equipment.

Also look at the size of your battery, electrically and physically. It needs to be about the same physical size as a standard automotive 12-volt battery, but with just three cells. There's half as many cells, but each one has to be twice as big to get twice as much current.

If the cables and battery are okay, there could also be a poor connection in the starter circuit. Most of the time that results in a no-crank condition, so we'll leave that for our next discussion, if necessary.