If the piston were used with a 4.25 crank & 7.100 rod, in a block that was decked to 10.700”.......it would be a 505 with the pistons down the hole .063”.
With a 78cc head and all the usual other stuff...... it would end up at 10.2:1.
Obviously, without a detailed build sheet it’s all speculation until someone takes a head off.