It's rather (2.6/2) * (2.6/2) * 3.64 * Pi = 19,32 Cu.in per cylinder (or 316.7 cc per cylinder). In case it is a four-cylinder engine, we would have 1267 cc displacement, or a little less regarding how many digits you take for Pi.
The displacement would fit a Standard Ten, but that was a long-stroke engine (100 mm = 39.3 in).
Then there is the Hillman Minx engine: 1265 cc, 2.56 × 3.74 in, which would come closer.