Thanks for e-responding and telling me the stove will remain in place......COVERING the hole.....
Since the repair will always be covered.....any tight fitting patch of the same thickness will do an adequate job of fixing your floor/hole.......
Normally if this was to be seen......you would break the flooring back to solid joints and work new floorboards into the opening to replace those rotting ones............
Here your interest is to seal the opening........a plywood patch cut to fit the hole and firmly attached to the remaining framing/sub-floor will 'fix' the floor .......doug