I've opened up a PET exactly like the one you pictured EDR. There is more than enough room to stick even a full ATX mobo where the old one is. You'd have to make your own mobo plate out of sheet metal, and have it mount to the existing mounts the original mobo was attached. Something someone with a ruler and drill could easily accomplish.

As for the keyboard, I'd remove the existing keyboard. Then, get a cheap modern keyboard, take it apart and mount it where old one was. Being as the old keyboard was missing a few buttons to todays, you might want to (or nearly have to, for the CTRL/ALT keys) cut a notch for each pair on either side of the spacebar to fit them. I'm not sure where you could jam the arrow keys, but I'm sure I could think of something.

The keypad's spot is no doubt farther away from the main keys than current keyboards, but that would be easy to fix. You see, most any keyboard made now a days uses contacts on soft, easily cut able, plastic. It would be easy to cut this soft plastic apart, without cutting the traces embedded in it (they are very easy to see). The keyboard's hard back (was a PCB in those days, now a sheet of hard plastic) must be mounted on the back of the newly reformed keyboard, to give the keys something to press agenced. Replace the keyboard's original keycaps with the PET's, thus giving the new "old" keyboard a great original look.

If you’d like, I could draw a nice “PET fit-able” keyboard layout, featuring all of the modern keyboard’s keys. Just PM me.

As for the monitor.... The PET had what? a 12-13 inch? If I were you I would locate a similarly sized (and working) SVGA monitor, remove all the guts from the case (only do this if you know what you're doing, as the stored power in the caps/flyback transformer could easily kill you, possibly even after the monitor has been long shut off and unplugged), remove the PET's original monitor/monitor guts, and move the new monitor's guts in. If only the tube of the monitor will fit into the original monitor cavity (which that might be the case) you may be able to put the monitor's controller board by the motherboard, as long as you properly shield it.

All cables would be routed internally to the motherboard. As for the needed external connectors, you may have to cut a slot on the back where all the wires could fit, and connect them by opening the cabinet (you may also make them fit to the back, for a more “original” look, using extension cords which where the ends would be mounted (using the existing mounts on the extensions) to the back of the computer, replacing those originally there.

The PET, or found in a pile of garbage, looked perfect. The original mobo was trashed. Never thought of modding a modern PC into it, but I wish I had now...