Ok, you want them to count-as centurions, but do they have to _look_ like centurions? If rules-wise they'll be allies of your orks, then why not model them as orky centurion equivalents? They'd probably look something like killa kanz, but more heavily armoured and with different weapons of course. Not trivial to scratch-build such things, but far easier than scratch-building a marine centurion.
Or the cheap approach would be to use those termies you already have, with some sort of bolted on extra armour and extra weapons, fluff-wise representing either an early prototype of centurion or an emergency battlefield modification.
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