It's clearly a two stroke and I wouldn't be surprised if it's not the original motor, something from Villiers perhaps. That rear sprocket looks modern and I'm liking the size of that horn!
1930 Triumph model X 175cc. 2 speed, Villiers 2 stroke engine Took some bloody finding as well, might I say!
BASTIDS!!!!! I'm sure I read somewhere in the course of my search that the Villiers engine was actually designed by Triumph. Gave around 1.75 hp apparently - some info on Triumph Owners Club site.