Can be done, but there are a coupla things to watch out for.

Alkaline batteries: Cell voltage 1.5V. Non rechargable. Capacity up to 2500mAH

Lithium batteries: Cell voltage 1.5V. Non rechargable. Capacity up to 3000mAH

NiCad batteries. Cell voltage is 1.2V and gets lower with age, as does capacity (memory effect). Self discharging. Bad for the environment.

NiMH cells: Cell voltage 1.2V. High capacity. Minimal memory effect. Self discharging. Good capcity to weight.

Li-Ion cells: Cell voltage 3.6V. High capacity. Minimal memory effect. Excellent to weight. High cost. Requires power management chipset. Individual cells and chipsets not available in Oz.

Best option is Li-Ion as they pack plenty of punch at about half the weight and size of NiMH, which is the only other real choice.

You have to match the voltage rating of the existing lead acid battery, so if it's 12 volts you'll need to build some sort of voltage regulator circuit if you use Li-Ion as your only option would be four cell (14.4V). There is no chipset or charger that I know of for a three cell combo (10.8V) Li-Ion

For simplicity go with NiMH. Chargers and cells are readily available from Jaycar, Dick Smith, etc. Get high capacity cells 3000mAH or higher. Don't try to get advice from the store tools, they wouldn't know resistor from a Van De Graaff generator