making a camshaft isnt that terribly hard to do. if you can make engine parts, you can make a camshaft... and timing is no real challenge either. just requires accuracy in laying out and machining.
theres not much difference between having two small pistons firing at the same time or having one large piston firing...same power. same pressure. more frictional losses though. you should get more speed from the smaller lighter pistons and therefore more peak power... but the torque will remain relatively the same. ever wondered why dirt bikes prefer one big cylinder compared to road bikes with two, four, or six cylinders? its actually a bit more complex than simple torque/power...its also about the smoothness of the power pulses. dirt bikes, with their thump thump power delivery can slip...then get a break while they regain traction...then slip...repeat process. watch an old lenz bulldog tractor working under load... you can see the jerks as the single cylinder engine delivers a power stroke. for the dirt, for traction, that power delivery was far superior to the smoother running multi cylinder engines.
multi cylinders tend to spin but the firing order is (usually!) arranged (on bikes) so you get one whole revolution with all the power strokes, followed by another revolution with no power strokes.
i always found that a v-twin, when it breaks traction, remains very throttle stable. very steady. whereas a inline four, as soon as it breaks traction it tries to spin itself up to redline...scary.
car engines are designed for smoothness of power delivery, all power strokes are evenly spaced. early bikes did this too but they soon discovered it wasnt the best way to design the engines.
not really an issue with low horsepower engines, but yeah...something to think about.
and balancing is harder, unless its horizontally opposed.
