I have an XMOS dev board sitting in a drawer basically unused since I found I had to use their proprietary extended C dialect. But I understand they may have fixed this since?
And you can include reglar h-files to easily link to and build regular C. With provided macros you can have h-files which are using some XC features but still be compatible with both XC and C files (making it easy to wrap XC code and call it from C and vice versa).
There is also this that can use XC features from C. https://github.com/xmos/lib_xcore_c
But I don't find XC to be bad, and likely a much better starting point than the library above.
The compiler is a fork from gcc from 2006 (if I remember correctly) and that shows its age somewhat. The many protections in XC can be a bit tedious as well.
Of course this is just another of my unfinished hobby projects, so :-)
Is your codebase uncompilable using their tools? Would a simple hello world compile? How foreign does a simple blinky program look to a C programmer?