Best Guess ?
Coil, Voltage Regulator, or improper grounding.
Check the grounding first with a DVM, since that's the simplest, and both the Coil and Regulator are like $50 parts.
Also try a different plug, NGK plugs don't work well with Tomos, use Bosch or Champion, preferably Bosch, and if you have to use Champion, avoid the canadian made ones, they are junk. (damn NAFTA anyways)
The Bullet series can be completely rewired to a frame-grounded system if the wiring harness is screwed up, you just have to insulate the regulator.
Most of the time, in a case like this, it's the coil, however.
Easy way to find out would be to swap the coil from the other one, and see if that works, also, being that they are both 78's, check the cylinder bolts for tightness and make sure you are getting good compression, that's always been a problem with older Tomos mopeds, eventual compression loss.
The reason is that the steel rods that hold the cylinder in are screwed into the aluminum transmission housing, and wear faster, not to mention being all too easy to strip out.
Plus you have an aluminum top and steel bottom to the cylinder, and I've always thought it unwise to use a metal-to-metal seal with two different kinds of metal, though in practice it does usually work.
Pull the exhuast baffles and clean them too, a clogged baffle is a severe detriment to performance, and can be cleaned by beating with a hammer or a little light blowtorching.....also, though not reccommended by the manufacturer, I simply pull the baffles entire and don't bother putting them back in, you get a very small performance boost, and it changes the exhaust note from the Tomos's normally wussified putt-putt-putt to more of a boom-boom-boom, which is more a style issue than anything else, but amuses me all the same.
Hope alla that's helpful,