If the code is only battle tested you have a problem. If you did the correct thing and have written the actual tests for each of the lessons learned nothing stops you from porting these over to Rust.
If you don't have the tests you just listed another reason why using a language with better safety guarantees that will not let broken things compile is a good idea.
The way I see it even battle tested old code can become a liability, if it's surrounding and dependencies change.
It's often impractical to fully test every single aspect of all code. An example that I've seen several times is bottlenecks in unexpected places. Once was due to false sharing, another due to worse standard library QoI.
If you don't have the tests you just listed another reason why using a language with better safety guarantees that will not let broken things compile is a good idea.
The way I see it even battle tested old code can become a liability, if it's surrounding and dependencies change.