Be careful fitting a replacement, they are finicky to get right unless you read the manual.
I broke the switch the first time I replaced mine, many decades ago.
The later series have servos, and a switch in the engine compartment. Mine is like that, not that exact type of switch, though. Surprised the bonnet closes!
That's a breather, it lets excess air out through that hole. Just paint it up, it's fine. But you can swap to the military version with a proper breather if you want.
Assuming it's petrol.
Could be evaporation of the fuel in the fuel lines.
Could be the fuel pick up pipe in the tank part blocked.
Or could be the coil breaking down.