A replacement keyboard for Commodore 64 has already been done. It is called the
MeC64 and there was a small group buy back in 2013. If you ask tlt nicely, maybe he'll share the design ... or do it again.
The MeC64's plate was bent in ways that made it recessed, to account for the height of 3D-printed adapters for use of Commodore's keycaps on Cherry MX switches.
Since then there has also been a run of C64-styled keycaps in SA profile:
SA Retro, complete with front-printed PETSCII symbols. Personally, I think real Cherry MX keycaps would be preferrable for feel as lower keycaps have less wobble.
Edit: Check also out the
GMK Retro: same colour scheme and legends but in regular "cylindrical" profile and high-quality double-shot keys.
BTW I read that the matrix is that small in the C64 to save on I/O chips. The matrix is also shared with the joystick ports somehow.