Adaptation of Tecdoc + Opencart script

Web developer services are required. We're drinking auto parts. Opencart 1.5.6 – Many modules are installed, and still in the process. At this stage, the engine is working clearly.
The turn has come to adapt the module for the selection and order of auto parts.
We have a module installed, the bases are connected and, in principle, works, but it is curved.
We need a backend and frontend adaptation for our site.
All work is pure php + mysql.
We are discussing the bugs already identified. The price is negotiated. I'm waiting for suggestions.