As far as I understood the squish area is important because it creates turbulence as the piston reaches top dead centre, which gives the mixture a more uniform distribution for improving combustion and minimizing chances of detonation.
Our cylinder heads are practically flat at the outer edge of the original combustion chamber and completely flat for the part between the 50cc and the 72cc diameter, so a 50cc gasket will change nothing to this shape compared to a 72cc gasket, except for a small extra ridge halfway into the squish area and a tighter squish clearance at the edges, which will only help increasing turbulence...
I wouldn't be surprised if the difference between a 50cc and a 72cc gasket on a 72cc cylinder isn't even measurable with a dyno...
The only two problems I could think of are that the piston might hit the gasket at TDC, and that the inner edge of the gasket (which has a narrow ridge pressed into it to compensate for non-straight surfaces) isn't supported by anything so it might leak a bit - but then again the same goes for dremeling this edge away...
edit : corrected some results of early-morning / before-coffee brain malfunction.
edit 2 : thinking of quitting my dayjob to try out everything for real that I have a theory about....
