Petrol leak ktm sx 126, year 2002, turn on the fuel tap and petrol starts leaking from one of the carb over-flow pipes (the one next to the choke). Why?

The float must be sticking, turn the fuel off lie the bike on it's side rattle and tap the carb and try that if it don't work carb needs to be taken off and cleaned

No, it is more likely a bent exhaust valve or burned. The fuel flow position may indicate an additional issue with the carb, but the exhaust should not glow red like that- when it does it means there is unburned fuel and burning combustion gasses entering the pipe.

If the bike is hard to start, this can be why as well. Do a static compression test to see if the valves are leaking.

Reseal carb / gaskets if its from the carb body ,

Sticky float, tap the float bowl to see if it stops and frees up
Check carb bowl for dirt in float valve or wear on float valve or seat or seat o ring, make sure the float is still floating (remove and try to float in petrol) if leaking from an over flow pipe or out let

1) This might sound daft but check if there is actually fuel in the tank. Many bikes have been stripped down by the hard of thinking only to find the tank was empty...or the tap was off... or should be on reserve... 2) Assuming this is a gravity tap (one main pipe) and not a vacuum tap (one main pipe and one smaller pipe at the side) Turn tap to off and remove pipe from tap. Turn tap on and see if fuel flows. If yes, then replace fuel pipe (this should sort it, try again with the pipe off at the carb end and see if fuel comes out. If it does but the bike still won't start then the carb jets are blocked.), if it still doesn't flow then go to3)
3) If there is fuel in the tank then remove tank and pour it out into a can. Remove tap from tank by unscrewing with the correct spanner. 4) Examine the guaze filter, it is probably full of crud and muck. Clean with fresh petrol and replace.
Hopefully that should sort it.

I would get a new hose and clips and replace it. It should be a fairly straightforward job unless there are things in the way.
The fuel hose is usually 1/4" bore neoprene rubber.

The float is stuck open. This causes the overflow and floods the engine.

first you need to remove carb clean them , then new gaskets . sounds like they are glazed due to old fuel drain tank and refill with new fuel

try lowering your oil/petrol mix ie 100ml oil-4.5l petrol (if premix), if not change spark plug to hotter grade (whatever plug number is go down a number)