The motor is likely a Villiers . Looks smaller because that's how 2Ts were made back then .Same as my Simplex 125cc :
My Simplex makes 4 HP and will do 50 MPH , on a good day . 40 - 45 comfortably .
The motor design didn't change much since the late 30s , but for a switch from rotary valve to a timed internal crankshaft intake . Not sure if that was the best move , but, much more simple .
Anyway , the Villiers are believed to be a bit better motor , so ...