做番老本行英文 (informal)

做番老本行 He's back to the job of his own profession 是累贅了, job 和 profession 這裡不應用在一起。 上面網友提供的 back to his old job 或 he's back to what he used to do 當然可以, 但如果不想直譯, 可考慮以下說法：
To resume one's old trade (trade 其中一用法是 ‘職業’、‘行業’)
To relive an old experience (relive=重溫)

He's resumed his old trade of selling fish.
He's relived his old experience as a fish seller.

兩者寫、說皆可。

He's doing what he used to do.
He's gone back to what he used to do.

I have a friend who used to sell fish. When he became unemployed he turned into a street vendor but now he's gone back to what he used to do - selling fish.

He is back to his old job./He is back to his old job of selling fish.

He is back to the work that he was used to be./He is back to the work that he was
used to be in selling fish.

His work is to sell fish that he was used to be.

He back to his own field