Tear down the CD-i until you can access the disc tray pulleys.

Here we can see where the belt should be.  I've removed the perished belt already.

The front panel needs to come off to remove the disc tray.

You can probably reach the pulleys at this point, but further disassembly is easy enough.

This latch must be moved to pull the tray out completely.

With the tray out of the way, now we can proceed.

Use a piece of string to measure the length of belt needed.

Measure the string.  This will be your inner circumference of the new belt.

I used 123D to draw up the belt, using the string measurement as the inner circumference.

I used a Lulzbot Taz 4, with the stock extruder.

Sliced with Cura Lulzbot edition using the PLA-fine configuration provided by Lulzbot.  I manually set the extruder temp to 230C, and no bed heat.  Be sure to prime the nozzle completely before starting the print, as Ninjaflex tends to ooze quite a bit.

On my printer, it was necessary to cover my bed with blue tape, because Ninjaflex can permanently bond to the PEI coating I have.

It may be necessary to iterate a few times, to get the correct tolerances (or if you miss the detail that 123D wants the radius for drawing a cylinder, not the diameter).

The other thing to watch for is belt thickness.  If the belt is too thick, the pulleys won't turn properly.