I went back through a lot of SAE papers and the transition period for professional automotive engineers to use motor and engine interchangeably in published works was 1910 through 1915.
Some examples:
1913-01-01
MOTOR CAPACITY FOR MOTOR TRUCKS 130003
DOI:
https://doi.org/10.4271/130003
Citation: Myers, C., "MOTOR CAPACITY FOR MOTOR TRUCKS," SAE Technical Paper 130003, 1913,
https://doi.org/10.4271/130003.
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Author(s): Cornelius T. Myers
Pages: 10
1913-01-01
MAGIC MOTOR * 130022
DOI:
https://doi.org/10.4271/130022
Citation: HUTTON, F. and CHASE, H., "MAGIC MOTOR*," SAE Technical Paper 130022, 1913,
https://doi.org/10.4271/130022.
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Author(s): F. R. HUTTON, HERBERT CHASE
Pages: 16
1911-01-01
A ROTARY VALVE GASOLINE MOTOR 110030
DOI:
https://doi.org/10.4271/110030
Citation: MEAD, C., "A ROTARY VALVE GASOLINE MOTOR," SAE Technical Paper 110030, 1911,
https://doi.org/10.4271/110030.
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Author(s): CYRUS E. MEAD
Pages: 20
1914-01-01
THE GENERAL TREND IN AMERICAN MOTOR DESIGN 140048
DOI:
https://doi.org/10.4271/140048
Citation: POWER, W., "THE GENERAL TREND IN AMERICAN MOTOR DESIGN," SAE Technical Paper 140048, 1914,
https://doi.org/10.4271/140048.
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Author(s): W. M. POWER
Pages: 7