Whatever you have to do to keep a bike running. General maintenance. There is a grey area there, if you get a guard re-chromed then that is restored but if you replace it with a VG second hand one then it isn't. If you only get one part restored then the bike isn't.
basically a good clean, maybe paint the frame, in your case upkeep, maintenance, rebuild brakes, clean motor, replace spokes, all maintenance and the bike would still be classed as original. Even putting a pamco in it, mainly because you can keep the parts you take out and the upgrade cannot be seen. That can be debatable though. Putting pods on and removing the air-boxes is visual so in that case it cant be classed original in my mind,. again debatable. Same with the exhausts, but because for me they are a defining feature, i would argue that the bike wasn't original.
look at the latest XSOTM contest and although they could be classed original they are more resto mod, royboy and blue900 have the most original, visually looking XS's but even then royboy has changed the handle bars and blue900 has an earlier model seat on it.
There are people who are advertising a restored bike, when all they have done is buy a well preserved bike and replaced things that are maintenance related and given it a clean and polish.
Case in point
HERE. This is what i would class original, liberties taken including the word restored, but i guess the owner/seller sees himself as doing the maintenance work to get it running and on the road as restoring it and he does state original/restored, but then the bike wasn't stripped to paint the whole frame so i think including restored is misleading in this case
In truth this will/could start a very vigorous debate and a lot of varying answers.