Try two or more gaskets, better still bite ya lip, buy new one and replace it each time
That comes from a bloke that at present has four base gaskets on his four stroke sidecar engine. Why? It is one simple way to adjust the compression ratio to various fuels octane ratings. Before reassembly, calculate required compression (That is the very technical part of your engines components you have to study and learn) and fit required amount of gaskets allowing for crush / tensioning of course. At present I can run 100 to 106 octane unleaded or avgas , 91 octane pump stuff would be (is..) crappy. The engine is so built and modified that one base gasket only, is the basis for methanol fuel.