developer to help make changes within our proprietary system

I'm in need of a developer to help make changes to our proprietary system with a mix of c# and vue. This is an ongoing project and I need someone who can analyze our system thoroughly in order to identify the issue/s. Whilst there is no strict deadline or urgency, it would be great to find someone who can help out quickly and on a regular basis. Our current developer built the system but we have some specific needs that are either beyond his knowledge base or he does not have time to complete them without causing other issues in the system.