Dodgeman wrote:

These engines use No. 3 main bearing to handle the end thrust loads [crank end float] so it works as a 2 cylinder, but imposes yet another engineering problem as a single.

As for engine balance as a twin, the primary balance would be OK [rising piston counterbalances falling one] but it would have a rocking couple. As No.1 piston reaches TDC it tries to pull the front of the engine up, while at the same time, No. 2 piston reaches BDC trying to push the back of the engine down [inertia at work here] so the engine tries to tip back. This process is reversed in the next half rotation.
At opreational speeds the engine will not actually rock, but there will be a vibrational wave in this direction.
Since this engine is mounted on the sides, giving it a lateral pivot point, this condition could be amplified to some degree. If it was 3 point mounted the vibration may well be less.

In the original 4 cylinder configuration, the center cylinders are counterbalanced by the end ones for primary balance, & no rocking couple exists as the rocking couple of each pair of end cylinders is opposite to, & therefore equalises, the other end.

Coming up with a solution to the dipstick problem would be the least of your problems in a single cylinder configuration.