There's many things you can potentially do at a loss of performance. E.g. train it on all the code initially to learn the general rules better but then finetune

There's many things you can potentially do at a loss of performance. E.g. train it on all the code initially to learn the general rules better but then finetune only on the subsection that is of the right license or write something into your inference that checks how close to existing code the output is etc.