The engine sprocket has a large boss which has two ratchet dogs milled into it which then engage with a matching pair of pivoted ratchet levers in the outer housing (keyed to the engine shaft). The engine sprocket also has two ball races to allow it to 'freewheel' when engine starts. Never got around to fitting springs so crankshaft sometimes requires turning so one ratchet lever engages by gravity, it then auto disengages under influence of centrifugal force.
The top shaft runs in a bit of pipe bored for 'oilite' bushes with collars each end, also has two welded on tags to fit to extended cylinder head bolts (nuts changed to double length type). Chain is tensioned by shimming with washers, if required.
Edit: Found a pic of the 'silver steel' ratchet lever pivot, pressed in with split pin, just in case etc.
Just a copy of the Lister clutch made from available bits.