Simplify the following text for a Grade 6 native English-speaking person.
Original: The first unit to be re-engined was D6123 , a further 19 machines were re-engined in 1964-1965 at Polmadie , along with other modifications including the fitting of four-character headcode displays in the nose ends .
Simplified: A further 19 were re-engined in 1964-1965 at Polmadie Works , with other changes made at the same time . This included the fitting of four-character headcode displays in the nose ends .