I have a complete system coded in C and was wondering if it were possible to use it for the competition some how (integrate via API?). Or would I have to rewrite the system in mql?
Also, is the competition done with *live* market data, or is it done with history?
You should read the rules.
Rule #1: "Programs are accepted in compiled files *.EX5, as well as in their original format *.MQ5. DLL-libraries are prohibited".
If your system is written in C then it shouldn't take too long to migrate it to MQL5, probably less time than trying to integrate it via API.
I'm sure the answer to your second question will be in the championship information as well.