Well, the chances are bigger than you'd think accutually. I have a friend who I definitly can see porting a game, especially if it's easy.

What I would basicly need in an engine:
- Ability to render in orthographic mode (no need even for 3d-models)
- Play a sound
- GUI system (!)
- Scripting system (AI, conversations, map triggers/traps/alarms, ..)
- Ability to use custom map format (or one with a format compatible with the design specs)
- Networking

My own game framework I've designed should be able to take care of most of those things. I know that I should be using an existing engine, but it feels like I'll either end up with spending even more time trying to figure it out and modding it to suit my needs, or that I'll end up with something far too limited, or worse, something just not usable/suitable for my needs, meaning I'd have to redesign some parts of the game.

I have written engines in the past, just never in C#. The two most challenging points tends to be physics and graphics. This game doesn't really have physics, and only very simple graphics (from a technical point of view anyway).

There are cases when using an existing engine would be just plain better, like when making an FPS, but I also think that there are cases where one would be better of just writing his own stuff. Using an existing engine just for the sake of using an existing engine isn't really wise imho. But then, I suppose it's not very wise of me to think I can do anything by myself either. :p