First of all you should download latest server files from github. You can find this here. You don't need to rebuild your script, because open.mp is fully compatible with SA-MP. If you want to build this gamemode on open.mp include without any changes in your code, you should replace include a_samp with open.mp and add SAMP_COMPAT before your includes. That's all.
Example here
#define SAMP_COMPAT
#include <open.mp>
//etc.