Hi again,
UUM identifies the bike as a 1954 Falcon Series II model 67. The engine number prefix for that model is 387A so the number as given is a bit of a mystery. Not to worry, Villiers engines are interchangeable and it is no surprise to find the engine has been changed.
It is not a trials machine no matter what has been done to it. The trials machine of that year had a frame number prefix of UTM and the scrambler equivalent was UTMS. Falcons for overseas used UMSO.
Cheers,