Boris,
As DickyD has said, it isn't such a big problem.
At some time or other most of us have done something like it.
Either just get a piece of plasticard and make a new part or get one from Model Slipway as FLJ has said.
With this type of problem take your time, if necessary cut out the whole deck or cut out part of it.
The operative word is CUT.
But you know that now.
You do have my sympathy but it is all a learning curve, you are just on a steep bit right now.
It will get easier and better, don't lose heart just yet. Getting over these LITTLE problems is all part of it.
And believe me it is only a little problem.
Bob