I would remove the bottom extended step pieces (where the lawn jockey is) and do a railing up the left side of the steps.

To use Trex or any composite will require the framing to be modified to 12" on center joists at a minimum to avoid the sagging of the composite deck boards. To to say on railing as we cannot see how the existing is built and railing posts would need to be attached to the framing below the surface and most likely buried in the dirt with cement and footings due to the shallow depth of the framing system.

Thanks. I will mention that to whoever comes to do the job.