Supposedly I have the training to do these things myself, but I sure as hell wouldn't want to try forging mods like these on my own.
There was a video game system available for a while--a joystick with a 64-on-a-chip inside it--and you could just plug it into a TV and play a bunch of classic C-64 games. Turns out that someone who knows what he's doing can actually convert the motherboard in that system into a fully-functional C-64.
One guy went so far as to build a C-64 laptop, with an LCD screen. Of course it doesn't run on batteries, but requires an AC power brick, but even so--nice job. Another guy took a Raspberry Pi and ran a C-64 emulator on it, in a C-64 case (with keyboard of course).
And this is one system. There are equally active hardware hackers who do their thing with Ataris or Timex-Sinclairs or Coleco Adams or Amigas or-or-or.
Then there are the hardcore guys who build their own damned computers from basic components. You really need to be serious if you do that. Someone's still making 6502 processors, and they're dirt cheap; and furthermore the C-64 is a simple computer, simple enough that a sufficiently dedicated person could build his own if he really wanted to.
(C-64 actually uses the 6510A, which is a 6502 variant, but you get the idea.)
...but there's no need to. A Raspberry Pi costs less than $50 and has more than enough computing power to emulate a C-64 in real time.
It's almost enough to make me want to set up an electronics bench in the basement and start doing my own tinkering. Damn.