Sovereign debt is very tricky. You can announce that you don’t recognize it and you have defaulted, but the creditor does not have to agree. It can seize assets, like ships on the high seas. This happened to Argentina.
As for having an income I was referring primarily to the transit fee of gas to the pipelines from Russia to Europe. Europe can refuse a price that has the transit fee built in.
What would a post war, pro Russian, Kiev government budget look like? It relies on Transit fee for revenue. It certainly will have taxes on businesses and people, but the spending obviously we’ll have to overwhelm that in order to pay for repairs.
Ukraine population was also in decline long before 2014. A declining population is not the formula for reduction of deficit.
The end of regime is the end of debt. Particularly if it become a part of Russiaand the debt was used against Russia. It will become a part of war reparations. Although I don’t believe that Ukraine will disappear compeltely. There will be a rump state in Galicia or a government in exile in Warsaw or London. Creditors will be free to sue them.