I'm betting there is a timing problem.
Pull the plug and do the straw down the spark plug hole trick.
Find T.D.C. by the straw and align the timing marks, then see if the valves moved by the cam are closed or open.
On my 660 and 700 the cam gear index mark have aways been off a little.
Sheared woodruff is a possibility. Could be a bad timing chain. Is there a lot of slack in the chain?
Double check that the engine was at TDC of the compression stroke when you checked the cam timing marks.
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