The standard Kymco Agility takes 50ah batteries and unless you have a larger box than standard you will not be able to physically fit them in the size of the 50's are here on the page, have a look at your box and measure to see if there is any extra room.
You can however upgrade to lithium, same physical size battery but you can use 100% of the capacity.
From new the Kymco Agility is quoted as being able to do 25 miles on a charge, if this was a benchmark then with lead batteries you could only do 12.5 miles as you should never run them down past 50%, over time this would shorten even further.
With Lithium however you can run it down all the way with no effect to the battery, so 25 miles each charge.
They are good for 5000 charges and come with a 5 year warranty, when you compare to lead they are capable of over ten times the use.
and you can use your existing charger, just swap them over with the lead, no other factors to take into account, a very simple upgrade, the Lithium version is available in the alternatives.