HI Duncan,
It is possible to see the pushrods sitting into the rockers if you remove the tappet covers and use a strong torch,
I found a LED one seemed better ????
Once it happened to me that the pushrod sat on the edge of the edge of the rocker cup
I could see that one tappet adjuster was way different to the others
When you say the tappet adjusters are very long, do you mean screwed all the way towards the valve or away from the valve????
A lot of times newly rebuilt engines or ones which have sat for a long time are hard or impossible to start on the kickstart ???? ???? ????
Have you put fresh petrol in it?
Once you are satisfied that the pushrods are ok, I would try push starting it, enlist a strong accomplice to give you a hand
HTH
John O R