Thanks all, I'll stick with the current setup, which is the tyga blank plug sealed with silicone gasket glue.

Yes, this is an mc28 engine, so I have turned a small plug up on the lathe and plugged this hole too.

Interestingly, the last engine I was running had been built by someone else. The plug was simply installed and held in place with no sealant. Only found out when I was pulling it apart. Amazing it ran as long as it did!