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Install gas heater

I had an old colonial gas heater in my lounge which had stopped working. It has been removed by a plumber and we've temporarily sealed up the flue hole until we got a replacement. I have sourced an old (good condition) Rinnai gas heater which I need installed however the back of it doesn't line up with where the existing flue is. I just want some advice to see if there are any options to successfully install this.