To get it to electric start.
I drove the alternator with a poly V belt so I made a pulley to give me the correct speed when the belt was driven by the flywheel circumference. This worked well.
I then made a V pulley and fitted a one way bearing aka sprag clutch from simply bearings. It has key ways in both the inner and outer race to lock it to shaft and pulley.

The poly V, fan and Sprag V pulley an the left of the pic are all on the same shaft. My alternator has a taper fit shaft so they were all held on with a nut.

The transit starter motor had the pre engage mechanism removed and a small V pulley added. Once started I flipped the belt off with a screw driver to save wear on the sprag clutch as it was running at 3000 RPM. The pulley is locked with a grub screw in a flat on the shaft.
The system worked well and would run the lister up to speed and allow the decompressor to be dropped out it would then start but if not had plenty of power to keep the engine spinning. I charged the battery with a mains charger.
I also removed the COV and replaced it with a bung with a glow plug fitted to aid starting on veg. The high pressure line is also heated electrically to aid running.
Bottleveg has fitted two doors to his engine room, one inside the other, and it has made it very quiet.
Btw it is bad form to leave the rocker cover on, it needs oil at each run! Looks nice too.
HTH
Let me know if you want more info.