Both PCB's are so easy to reverse engineer that it's barely worth attempting to find info on the development boards.
https://duckduckgo.com/?t=h_&q=microwilli+development+board&ia=webhttp://d.pcnews.at/ins/cds/009/_pdf/n740060.pdfFor the atmega16. I've got a few of those too, but my preference for development boards is to use something breadboard compatible. You can stick an atmega16 directly into a breadboard, but for more reliability, it's good to lift the pins for the crystal and solder the crystal + caps directly to the IC. If you run it from the internal RC oscillator, then this is not needed.
You can get away with putting the crystal on the breadboard too. In that case, there is no need to add extra capacitors, and it is wise to not use the I/O pins next to the crystal pins. Switching of those I/O lines will cause occasional lockups of the uC.
Getting started with an atmega16 is pretty simple If you get a datasheet, the compiler package, and a programmer you've got most you need. I always liked the USBasp programmers. You can get them for a few EUR from china. "Official" programmers from Atmel have always been quite expensive. I almost bought a "Dragon" a long time ago, but it was not designed properly and it tended to fry itself when the USB voltage was low. I have not kept up with Atmel / Microchip after the merge. The atmega16 is very much like the atmega328. It mostly has more pins, and atmel can't settle on the peripherals, so timers and such may ave a few more or less configuration registers or bits.
If you want to do something with the Microwilli, then you need an Eprom emulator. They are not so difficult to make yourself, but it requires a lot of wires because you have to buffer a lot of signals. With a few tricks you can reduce this: Use a RAM ic and then connect all the pins directly to a microcontroller (such as the ATMEGA16

) Then you can program the uC pins to High-Z when the SAB .517 is running, but you also need something to keep the pins (both from the RAM chip, the hc573 and the SAB..517 in High-Z when your uC is loading data into the emulated eprom.