I don't mean to be an idiot, but have you tested visually for spark? I've had good plugs go bad suddenly with no explanation.yes brand new bbr cdi and coil box
you have to go through the list one by one, as they have been mentioning above.
air, fuel, spark and compression.
spark is easiest check, then check that your fuel flow is adequate. look at flow to carb, disassemble carb and clean. compression can be tougher, I couldn't start an engine and had the fuel and spark. digging into the engine I found the seal behind the magnet had a little oil weeping through. changed the seal and the engine ran again.
for an air leak and you can't run the engine to find it, maybe suspend the back wheel (ratchet straps on the chain stays is one way if you can hang it somewhere.) so you can crank the pedal by hand and spray carb cleaner or similar while cranking it to see if it helps
this is just the way I go about the same issue, I'm sure more experienced people will have easier ways