The current app is huge and has a lot of legacy/messy code, that still running, changing it will be really hard now. Tests takes ages to finish, the initial pro

The current app is huge and has a lot of legacy/messy code, that still running, changing it will be really hard now. Tests takes ages to finish, the initial project structure is bad, many mistakes were done in the past. We are planing to build huge new feature, that can has it own UI, So we decided to go for SPA, but where are asking ourselves what's the best way to build the API, Just build it on top of the current project or new project with Rails or other stack ?!