I wouldn't mess with the head gasket on an o-ring head. Head gasket is a higher pressure seal and will be more likely to leak if you do some janky shit.
base gasket is a lower pressure seal so you can get away with more jank.
going to a thicker gasket (the cardboard from the back of a high-quality legal pad is really good stuff, like 2.5mm thick- 2mm compressed) will raise your exhaust and transfer ports, which has a similar effect to porting them.
some people call this a 'rev plate'... its not ideal, much better to do acutal porting, but it is 100% reverseable and lets you see how raising the ports 1mm or so will affect your powerband.
using thick cardboard is more likely to compress unevenly if you don't use a torque wrench, which can bind your piston and smoke ya. You can also cut a plate out of aluminum and sandwich it between thin gaskets, or anneal it and use motoseal, or any number of things to raise the cylinder a little bit.
it will also drop the compression on top.