Hello Emilio,
Lovely bike you have there.
Disconnect the magneto earth lead, this will eliminate a wiring / switch problem.
Try starting it with the reserve tap on.
You can save yourself a fair bit of uncertainty by taking the plugs out, with the bike on the centre stand you can put the bike in gear and rotate the rear wheel.
Stick your finger over the spark plug hole and feel for the four cycles on each cyclinder, suck, compress, depression then exhaust, this can check your CAM timing, valves etc etc, a broken exhaust or inlet valve is unlikely to cause your bike to die, will make it sound like a chaff cutter.
If all appears ok, squirt a little fuel (or use a spray can of ether starter, "start ya bastard" or "aerostart" ) into the throat of your carby and try to start it, if it fires you have a blockage in the fuel system.
There are a few fuel filters possibilities, could be blocked fuel tap filters, I have the same bike as you and it has a AMAL 376 (not standard), it has a fuel filter in the banjo fitting.
If none of the above works, pop the cap off the magneto, whilst turning the engine, check that the magneto shaft is turning and that the points are actually opening, points sticking shut will give you a dead cut, as will points stuck open.
Is your magneto tight, is the timing correct ?.
Dead stop without crunching and grinding sounds is a reasonably good scenario compared to what could happen.....
Good luck.