Morning , I used these people for my diamond washer & woodruff key .
https://www.heritagepartscentre.com/uk/ The engine is the same as the Audi AEL 5 cylinder ( think that is right ) and was used in the VW T4 vans . Nice people were very helpful and prices very reasonable .
From experience of this problem , if the key has sheared , you will be removing the cylinder head & replacing a couple of valves ! . The engine is an interference fit & the pistons will bend the valve heads over .
At the very least I would order new crank bolt , woodruff key & crank sprocket and diamond washer ( kid you not ) The washer is most certainly required to stop any slippage / movement of sprocket & damper on crank . The bolt you wind up to 120Lbft & then turn through a further 180° and that takes a lot of doing . If it has not got the washer or been tightened fully , you end up back at your starting point
hope this helps