According to this about $1 trillion dollars of Russian assets have been frozen by the international community
This money is currently 'frozen', not 'taken' i.e. there's an expectation that it will at some point be given back once Russia starts behaving itself.
By what mechanism might some (or all) of these assets be allocated to Ukraine as war reparations?
Currently Russia is supported only by Syria, China and Venezuela. Most of the rest of world is either firmly in the Ukrainian camp, or discretely keeping quiet.
War reparation's have been imposed in the past, but have always been future-dated (by this I mean the country penalised was told "you need to pay war-reparations of $xxx over the next ??? years") and they always required the agreement (albeit reluctant) of the country making the payment.
Bearing in mind the old axiom 'possession is nine-tenths of the law' and the overwhelming majority of countries against Russia, can't the international community just take the money and give it to Ukraine to help re-build their country?