If you are a convert / revert to Islam after your parent(s) have died can you do anything for them in terms of supplication or other forms of 'Ibadah to perhaps save them from the Fire or is it too late?
There is nothing to be done anymore. Maybe they had not heard of Islam and thus will be tested on the Day of Judgement but you can do nothing about it and nor can you remove their burden from them.
You cannot seek forgiveness for them:
The Prophet and the people who believe may not plead for the forgiveness of the Mushrikeen, even if they were [their] relatives, after it has now become clear to them that they will be the inmates of hell.
About this Ayah:
(At the death of his uncle Abu Talib) Allah's Messenger (ﷺ) said, "I will keep on asking Allah's forgiveness for you unless I am forbidden (by Allah) to do so." So Allah revealed (the verse) concerning him (i.e. It is not fitting for the Prophet (ﷺ) and those who believe that they should invoke (Allah) for forgiveness for pagans even though they be of kin, after it has become clear to them that they are companions of the fire (9.113) (Bukhari)
The Messenger (SAW) was forbidden from asking forgiveness for his uncle by Allah with the above ayah.
It is not your burden to bear. What they have done is what they have done. Allah will be Just and Merciful InshAllah, but their actions are their burden:
Say, ‘Shall I seek a Lord other than Allah, while He is the Lord of all things?’ No soul does evil except against itself, and no bearer shall bear another’s burden; then to your Lord will be your return, whereat He will inform you concerning that about which you used to differ.
It pains me to say this, but you really cannot do much for them. They have done their actions, and they will be judged justly for them.