99 times out of a hundred, an old bike that's been sitting around suffers from a carb / fuel delivery troubles. Eliminate this possibility before suspecting ignition or compression problems.
Start with a new spark plug.
Examine the condition of the tip of the spark plug after failure to start. Is the tip wet or dry?
If it's tip is all wet it's either fouling on too much fuel or the spark is too weak or it's just an old dead plug.. Weak spark can be cause by lots of things, from improper spark plug gap to some fault with ignition wires or components.
If the plug comes out dry, no fuel is getting into the cylinder.
Spray a bit of Starter fluid into the carb's intake or spark plug hole and reinstall.. if it fires up it tells you spark exists but no fuel is getting in.
If Starter Fluid doesn't fire it tells you spark is lacking. Starter fluid (Ether) instantly vaporizes and will fire at sub-zero temperatuires without compression so that test doesn't tell you if the head bolts are so loose you don't have any compression.