They need to be forced out, drill a hole side to side of the fitting, put something across and use it to lever them out (and turn them around at the same time)
If you're struggling to do it that way, you can drill them out by using a drill bit very close in size to the OD of the adapter (tiny bit smaller if possible), paying much attention to drilling as straight as possible and try to avoid any contact with the cam cover body
When the strength of the adapter is compromised that way, it will be easy to twist and pull it out
I would suggest thread based connectors, It should be easy to tap a thread into the cam cover and it will be very secure
Joe