To create a pre-trained model for C#
Welcome to the site - great first question! 

In your scenario (assuming I understand it correctly) you have a saved model in python right? So, what I would do is create a new API layer (written in python, using something like the Flask framework) and I would create API endpoints for your algorithmic functions (could be as simple as just 1 for your predict function). 

From there, you could run that python script on a server and expose that API to the outside world and it will run/respond just like any other REST API service. At that point, it would be fairly trivial to write C# code to access the API (or any other language you want).