I never use the timing marks on Villiers engines. I always set timing by setting piston at correct position eg 1/8″ BTDC for a MK15, then set flywheel so points are just opening. Checking several times during tightening of flywheel to make sure it has not moved.
The secret to stopping flywheel slip is degrease and clean both parts of taper then tighten flywheel properly. Villiers supplied a special ring spanner for this with a large lump on the end so it could be hit with a hammer.
Valuations are always difficult due to so many variables, such as condition and even location. Options include our own club magazine, e bay, mainstream magazines eg Tractor & Machinery or auctions such as those held at Tractor World show.
It would be logical to use an off the shelf item when manufacturing these machines rather than have a bespoke item made. If it is Morris Minor it should be easily sourced and not too expensive.