Just a cursory observation:
In the picture I can see above, the batteries have very small wires "pigtailed" out of them. Don't expect to run a starter motor at it's normal 120 Amp draw through wires that small. Starter motor cables need about 8 gauge minimum for the entire path (loop) that sees starter motor currents. That's both from and to the battery plates internal.
The picture also shows 2 - 12v batteries. You can parallel wire these to maintain voltage and double the current >>discharge<< capacity over just the one battery. However, batteries in parallel don't >>recharge<< evenly when configure this way. One of them always gets fully recharged before the other, and one of them steadily weakens. It may appear to "work" for a short while. But eventually, one of the batteries will stop giving full power and have to be replaced. Then the older batter will be the burden, yada yada yada. So, you'll have to replace the other one, etc., ad nauseum. Or, provide a mechanism to recharge the batteries independently after separating the common output connections.
If you are going to try multiple batteries, add the voltage not the current capability of each one, and wire them in series, if you want the bike to recharge them properly/reliably.
If you want to "electric start" the bike, the battery terminals must have some serious/substantial connection terminals on it.
Further, you'll note that the stock battery is "vibration isolated" from the frame with rubber grommets and rubber cushions. These isolate life shortening vibration from the battery internal cells.
I know many "cafe racers" won't survive more than a few years. But, if you want battery longevity, you should at least consider vibration, even if it is only the high frequencies, or the higher frequency harmonics.
There is a big difference between assembling components and "engineering" a solution.
FYI
