How to 'crop' a g-code

[paulus.v]

Hi all,

I'm trying to figure out a way of cropping a g-code. There are only G1 lines, lot of them, and I need to eliminate the lines that are falling out of a given shape in the XY plane. I do not care about safeZ for the returning or any fancy features, just eliminating the unnecessary movement outside of the workpiece.
Generating a separate toolpath for each shape is time consuming... Not to be fooled by the simple appearance of the example, there is a lot of Z movement as well.

Any help is greatly appreciated!

Paul