As a rough check both valves will be closed at the top of the compression stroke. This would indicate the cam timing to be correct, but I'd check the tappet clearances anyway. On a normal magneto bike the pick up to the cylinder on compression will be above the brass segment on the slip ring. With a Wassell unit I don't know how you check for correct spark timing, unless it's with a strobe. There must be someway to check the static timing, I would think, to install the ignition unit timed correctly.
Common errors are to time up after TDC and on a magneto bike swapping the leads. Is the Wassell a double ended coil? In which case it sparks in both cylinders at the same time.
So, if simple checks don't solve it, looks like the covers come off and it's back to basics with the dashes and dots.
Swarfy.