I have got to do the exact same thing with my K Series 1.8 VVC, plus change the belts etc.
May I suggest that you only purchase a genuine Rover Head gasket. There are a lot of generic head gaskets out there that are based on the original Rover gasket design. And to put it bluntly they are crap, as they blow through the moulded seals. Rover's latest (now last) incarnation was circa 2004.
This is much better than all previous types. And yes you can still buy them (and all other Rover parts) from X Parts:
http://www.xpart.com/
The Rover/MG parts business was sold off to Caterpillar logistics a year before Rover went bump.
XParts have restarted production of all current Rover parts. So it is good news to all Marlin 5exi builders/owners.
Hope this is of help to you.
Mike