Not knowing the history of the bike or of work performed, or whether it came from a bike museum or got pulled out of a landfill, some basic suggestions to give you a fighting chance.
First I'd inspect and clean all wires at battery and charging looking for any sign of corrosion. Oftentimes a wire at battery lug connection may look okay at first glance but peeling back some insulation reveals ugly green gunge infests all the wire strands. Replacement is the only cure.
With a fully charged battery connect a meter between disconnected battery ground wire and clean bare metal chassis ground, with switch off. Check for voltage flow and if any voltage is shown then you have an IOD, Ignition Off Draw, which is sapping power while bike is shut off. That means there's a short somewhere or a failed component.
Many switches can be dismantled to clean the copper contacts to restore life. Also closely inspect switch electrical connections, which are prone to failure.
Starter lead wire connections can cause plenty of problems just from a little corrosion or loose terminals. Also on wires, inspect closely where terminals are attached to see that all strands of wire are still in good shape. Sometimes a wire will look okay but close inspection reveals that most of the wire strands are broken through right at terminal crimp.
In many years of electrical troubleshooting and repairs I've found that way over 50% of 'electrical problems' are actually Ground problems, usually caused by corrosion. Best to disconnect and clean to bare metal on every ground connection on the bike if you want it fail safe for happy rides.
On the battery, some will take a surface charge and appear to be just fine and dandy when in fact they won't hold a deep charge. Charge battery, then put some load on it and see how well it maintains its voltage.
Basic timing issues, plugs and fuel delivery will all effect starting. I'd start on that by checking all plugs, then timing to see what's up there.
EDIT to add: sometimes a combination of faulty switch, faulty start pushbutton, bad wiring, ground faults, weak battery etc. can all occur at once like lemmings all jumping off the cliff together. That final 'one too many' is the straw that breaks the camel's back. Therefore I wouldn't stop troubleshooting just because you manage to get bike running. As long as you've got your sleeves rolled up go and check all the basics so you know right where you're at.
