In the end, I only had to apply the following patch to fix lack of header files:
...as well as modify mm_MYTH_TRUNK_VERSION in minimyth.conf.mk and also follow per this post by Pablo:
It works well - I had to use this old version to be compatible with my existing backend. And I was using an old MSI motherboard with onboard Geforce 4 video, so needed the nvidia 96.xx.xx driver.The mm_MYTH_TRUNK_VERSION only controls the SVN version for trunk. The SVN versions for the fixes branches are not expected to be changed as often. As a result, they do not have a control in minimyth.conf.mk. To change there SVN version of 0.21, you need to change MYTHTV_SVN_VERSION in script/myth-0.21/package-api.mk
Thanks Pablo, it all works quite nicely.