I had a case very similar to yours only the code wasn't even using Struts. What I did instead was target areas that were particularly crappy AND causing a lot of problems. This targeted approach got us incrementally better.

Over a 2 year period we worked on refactoring bits and pieces alongside enhancement work. We always worked a 'Hardening' task into a project plan. By focusing on the specific areas that didn't work well we got the most bang for the buck. The stuff that worked we left alone. Also critical is that this work was done in the course of normal development and was released. The problem with a big rewrite it you go off for a year or more and then by the time you come back every things changed anyway and some of the nasty bugs got smoothed over and you lost your ROI.

We never did re-write the whole thing. We did stop using that platform for new work though, and pitched a new fresh platform for a big new project. That got approved and we delivered a great product in a reasonable amount of time.