Even without documents making something similar should not be hard - establish a protocol and working communications method (and note that even “Fast” I2C at 400Khz clock is still relatively slow) then it’s just a matter of protocol - my ATmega system is very simple: command byte followed by a number of 32-bit values then it’s just the standard (gcc) C math library that does the work. (It has a shared memory style interface to the 65816 so relatively fast)
I wonder that with todays microcontrollers - is it needed though? However for emulation in retro projects, maybe there is a case there… but then, I suspect you’d be looking at a bus level interface (rather than I2C, SPI) so that then might lead itself to using a CPLD/FPGA type device.
Not really. With hundred+ MHz class 32-bit micro-controllers being available for very small amounts of money (witness the Raspberry Pi Pico, and the fact that the entry-level Arduino Nano Every uses a ARM Cortex-M0+ to manage the USB connection of its much slower ATMega main controller), the market has moved on. Most of the new chips aren’t 5V tolerant, though, so interfacing to older logic has an extra layer of compliation
For me, a 5v chip in an 8 pin through hole package is still quite an attractive retro option - with retrocomputing, with me, it’s very rarely about what’s needed (or even what’s cheapest) it’s about what’s interesting, and what has some feeling of authenticity.
It looks like the uM-FPU was well documented, in terms of interface and function, so a remake with a suitable tiny serial-capable microcontroller could be interesting. What it would take is some coding, which is of course not everyone’s cup of tea.
But everyone - indeed, each project - has their own value system, ranging from running emulators to building with diodes and transistors. There is no ideal, or optimal, retro project.
And sometimes it is interesting to to keep things about certain period in time. For me that is some thing around 1976-1978 with memory and logic devices. As for pure retro is the floating point rom
still around for the 6809?