Re: 1960's rollfast...
those bikes are pretty much junk. i mean, they're cool for pedalling, but put any stress on them and they'll break. i don't think mine was just old and weak, i think the design just doesn't work wit the added stress of a motor.
worksman's are strong, but ugly. there's plenty of old bikes that'll adapt quite well with a motor, so if you want the old timey look, you can stalk ebay or craigs til you find one.
but if you like the look of a worksman, you can't go wrong with it.