Depending on what speeds and weight of rider you plan on operating this conversion bike.
1" headtube for old road bike frames may not be ideal for conversion candidate to begin with.
1" steertube diameter fork may not be ideal for much motor power output & braking force required.
Old road bike fork may not even have the hub spacing that can accept a motor.
Better off leave the old vintage road bike as it is, convert a more modern mtb, or better yet just buy a complete ebike for under $1K.
Adding 15-20 lb. to the old road bike with motor, battery, controller, etc.. I wouldn't do it.