If you want to use C# you need to run Windows either XP emmbedded or WinCE. Since C# is a fixed requirment. No single chip CPU will work. The smallest proceser would be a 32 bit ARM. What you need is a single board computer with a version of windows pre-installed.
since this is an 8bit 8051 board you are lost.
You need to find a group that covers that subject.
try googling for Windows single board computers.