kz400 battery ?

Here's what should be a fairly easy question that I just happen not know or know where to get the information. My project is turning a 78kz400b1 into a small cafe racer. I have yet to start actually tearing into the bike because i'd like to get most of the parts for mock up.
My question is I want the bike to be a kicker only. But i realize that these bikes are designed to have a battery. If I don't plan to use the starter and only really plan to have the ignition, head and tailights operational how small of a battery can I reasonably go with without running into problems.
I was looking at the powermax sealed AGM so i can mount it on its side beneath the seat.
Model #
GT4L-BS
12 Volt-CCA 50 4 AH
Length
=4.50"
Width
=2.81"
Height
.38"

Any feedback would be greatly appreciated.

Well my friend, you can get the same size battery or something smaller but it HAS to push the same amount of amperage to the coils, lights and other electrical accessories, besides your starter,but I think 2 starters are better than one