Someone did advise me that if using the string method it is better to use hemp rather than man made fibre as hemp will burn rather than melt should any break off inside combustion chamber.
Also worth checking good contact between coil and HT lead where the two meet in cap. There is a tag on coil which makes contact with HT lead connection in cap.
You will need to register it, see FBHVC for guidance and DVLA website. For age related number plate DVLA will need proof of age. If unmodified it would be tax and MoT exempt as it would be historic class.
David, the Museum of English Rural Life (MERL) at Reading hold factory records of machine serial numbers from which you will be able to date your machine. They also hold original engineering drawings. Follow above link to see their website. A search of their database for Allen Scythe gives these results