Well....
In true Stationary engine parlance the front of the engine is where the power was taken from, the crankshaft. We see this end as the back these days! Preferring the " gubbins " to look at.
Many of the larger engine manufacturers gave the end user the option of running direction by providing the means to reverse the valve gear.
In the case of your engine Willy, there shouldn't be any need to reposition the cams on the sideshaft but simply drop the half time gear away from the crankshaft and then rotate the crankshaft by 180 degrees. Re check the operation of the valves, inlet starting to open at TDC being closed just before or on BDC. Both closed at TDC ( ignition ) exhaust starting to open just before or on BDC. Obviously in the reverse direction to what you had before !
Cheers Graham.