Alberto Bonelli of Durango manufactured auxiliary bicycle engines from 1948 to 1953
In 1948 Bonelli began producing single-cylinder 48cc two-stroke clip-on engines which mounted below the pedal crank and drove the rear wheel by roller.
These were followed by a 2-speed version in 1950, the Colibri 45, which could cope with quite steep inclines without resorting to the pedals.
A scooter was produced in 1951, the "Febo". Financial restraints forced the sale of the design - more information: Febo. Production of the bicycle engines continued into 1953.
Sources: OTTW, et al.
N.B. Several firms used the Colibri brand.