A gaseous fuel engine includes an engine housing having a cylinder block with a combustion cylinder therein, and an engine head including an intake port, an exhaust port, a fuel port, and an igniter bore, each extending to a fire deck. The gaseous fuel engine includes a fuel admission valve movable to close and open the fuel port. A non-axisymmetric clearance is defined between an inner port surface and the fuel admission valve and directionally biases a flow of gaseous fuel from the fuel port. Related methodology is also disclosed.