There are amazingly cheap ($2 shipped) STM32F103C8T6 modules on Aliexpress with the STM32F103C8T6 MCU which features 72MHz clock speed, USB, and CAN. The modules come with a RTC crystal for super low power time keeping.
References:
These boards seem to come without a bootloader. If you want to use STM32duino, flash their bootloader with your favorite SWD debug adapter.
I used a CMSIS-DAP adapter and openocd:
openocd -f interface/cmsis-dap.cfg -f target/stm32f1x.cfg -c init -c halt -c 'flash write_image erase unlock "generic_boot20_pc13.bin" 0x08000000' -c exit
The STM32duino puts I2C SCL on B6 and SDA on B7. Unfortunately this is poorly or not at all documented. So here it is. Source: Wire.h on github.