[squirrel_hunter]

I've not experienced this myself but it sounds a little odd that it started and then wont turn over the following day even with the spark plugs out and turning the rear wheel. If the engine is suspected to be stuck then while I would think to test directly by turning the engine from the fly wheel as done to set the cams but that might not be the best thing...

If you were in the carbs then I'd go there and start looking. Take the carbs off and see if there is anything trapped in the inlets. Check that the hoses are all still correctly connected and the balance pipe is blanked back off. Do you have a bore scope or way of looking into the bores while the plugs are out to ensure that there is nothing in there? I'd also be tempted to take the valve covers off to see if there is anything obvious there in case there is a stuck or bent valve or the cam chains are still attached and tight. Hopefully this will be a simple fix to a simple problem but I expect you'll need to do a bit of time consuming investigations and take it slowly step by step.