I am not so sure it has one.
It seems more to me, that there is a limit to how far the timing will advance on the stock CDI, and this would function to some degree as a rev limiter, yes.
Yes, the Tomos dirt bike CDI should fit, being a compatible system, but I have not tried to personally....I believe the MX50 or Bt50 ignition is what was mentioned above.
I dunno about pulling one from an old non-tomos dirt bike, but I suppose that might well be possible, check your local salvage yard and take a personal look-see.
(CT-70 is worth lookin at)
You must also remember...with all the "power" mods on some of these Tomos mopeds, you might be pushing nearly (or in some cases more than!) 8000Rpm, and trying to push past that is just begging for trouble....most stock engines of ANY kind start doggin at 7000Rpm, so in practice that limit is something you do not wanna push too hard...well, unless you WANT to see a Tomos throw a rod.
If that happens, send us a pic...cause it's "theoretically" impossible.
(But then, so was jumping one offa bmx trick ramp.....)