I used what Greaseweazel project refers to as the F1 "direct-connection" board. It is a STM32F103C8T6 chip also known as bluepill. However, I was not able to source this blue pill board locally, so I used another one (no name) which has the same chip. The pinouts are located elsewhere on the board, but the functionality is the same.
I did not use the recommended firmware upgrade process (
https://github.com/keirf/Greaseweazle/wiki/Firmware-Programming) because I did not have the special ST link programmer. However I was able to program it via a generic usb serial (3.3V) adapter I had, together with a programmer called STM32CubeProgrammer.
Also, through trial and error I found out that the latest g/w firmware 24 does not work with fluxengine, instead we need to use 2.2.
At this point I am trying to figure out how to write custom mini vmac images, as these do not have a header, and fluxengine is complaining about this. Diskcopy 4.2 images that I downloaded from the internet worked fine