Smallest battery for kickstart only kz650?

I want to hide my battery on my 1977 kz650 but I'm not sure what to go with. I know about the antigravity 4 cell lithium battery but I was shooting for something a little less expensive. All suggestions are appreciated! Thank you!

I use an antigravity 8 cell and a Ricks lithium RR works great.
The 8 cell is small enough to hide for sure.
If you want to use only kick start a 4 cell will be more than enough if you have a good stator and RR.

Cheapest route would probably be the smallest sealed lead battery you can find and keep stock charging system. Lithium requires different voltage set point

Any small 12v scooter battery should do it. They don't even have to be in very good condition. It just has to have enough juice to energize the field initially, then after that the charging system takes over and self-energizes the field.

I've used nearly-dead scooter batteries on 77 650's and Kz400's which use a similar charging system.

If you go with lithium, like waktaylor said, you will want to make sure the regulator is very precise and working perfectly, and make sure the wiring is in very good condition so the battery voltage is correctly controlled by the regulator. An adjustable regulator might be a good idea as well so you can lower the battery voltage a bit. The battery does not need to be 100% charged to get the field coil energized, so you can run the regulator at about 13.9v or 14v instead of 14.5v in order to have a safety margin so you don't overcharge the lithium battery.

I'd try a 12v Alarm battery. $24 @ Amazon
3.5x2.8x4"
www.amazon.com/Casil-Genuine-CA1240-Alarm-Battery/dp/B00UTY1IBW

FYI....if you have for example.. a 79 kz650. kick only and u used a 4cell AG battery.......it will swell and explode...
ASK ME HOW THE F* I KNOW.

on their site they say they have no battery for old bikes(meaning its a nogo)
they state in their warranty terms. no high heat. no old bike. no going(extra) small battery. overcharge and battery swells.
they will not warranty it and they know..they aren't dumb.

so if you want one of these itty bitty batteries you need that fancy regulators.

Dang, that sucks. I stick with sealed, lead-acid, AGM (absorbed glass matt) batteries.