I have been gifted a (broken) child's electric tricycle toy. The main problem is that the 6V 4Ah lead acid battery is defective - a collapsed cell I assume. I would like to replace this battery with Li-Ion batteries, specifically the 18650 type. I am thinking of using two 18650 to get the 6V needed (it will be a bit more at 7.2V but that will just mean the tricycle goes faster - Yeah!). However, I am finding it difficult to find suitable 18650 batteries and associated charger. It is not clear what the capacity of these 18650 batteries is. Do they vary? What is the maximum capacity? I have seen 3300 mAh mentioned but I'm having trouble identifying these batteries. I could create a parallel series arrangement of batteries to get higher currents but then I do not seem to be able to find a charger. I'd like to have separate 18650 batteries so I can use them in other things such as mobile robots.
I thought of using a 7.2V battery pack as used in cordless drills, but I cannot find any at 7.2V (or is it 7.4V - but I cannot find any of them either), plus I'm reluctant to spend a fair bit of money without knowing if it would be suitable or not.
Any advice or sources of information would be welcome.