Another option is to just start coding everything from scratch. The easiest way would be to start with Mbed online tools and then export your project. With or without Mbed, you can use the free open source GNU GCC tools to build your programs, which are the same tools we use to build the TinyCLR OS firmware. Another option is to use a commercial compiler like Keil compiler/IDE. Keil tools are free for 32 KByte or smaller program size (MDK-Lite).
If you are not adding a SWD-JTAG tool, like ST-Link, follow the STM32 bootloader instructions on generating and loading DFU files.