To get the machining done for O-Ring you would have to pull the engine out and a complete strip down.
Yes, you can fix the BHG insitu if you use an OEM HG.
You may also be able to do insitu with a copper HG, but you'd have to check with dominator. There will be thickness and softness issues I imagine. May be better than an OEM repair.
If you go OEM my experiences tell me there are two gaskets made by AJUSA, one only has the really thick firing ring - this is likely to be the one you want. Available from Fabre around Lakemba or Robbos Campsie even cheaper.
Michael