Wilhelm,
From a UK point of view ( I'm not sure of the Swiss situation) it seems to me that there are two issues involved in driving the car
The first of these is the legal/ insurance issue because cracked windscreens can attract the attention of the legal system here
The second is from a safety point of view. If the hood is not erected then clearly that means there will be no tension/ stress from that source on the screen. The issue therefore becomes one of stress/ twisting from other sources acting on a screen which is already weakened.
Two problems immediately 'spring to mind': the force of the airstream against the screen ( which you can, to an extent reduce by driving more slowly) and flexing via the chassis ( an obvious issue in a Morgan because the chassis, is of course, flexible)
You might try running clear tape over the crack or using one the the several 'glues' which are designed to be spread over the crack. Both of these are, naturally, very temporary measures.
Regards,
G
