you don't need a negrini magneto. you need one for whatever engine you have. it's either a morini or a minarelli. so you're in luck. those are very common engines. esp if you have a minarelli engine.
contact handy bikes, or moped junkyard, or akron mopeds, or any moped parts dealer (see the resources section of the site for links). they can get you a magneto. you'll probably also need a flywheel puller (unless you have one already).
also, are you sure it's that you need a magneto? not just cleaning the points or setting the timing better? magnetos, from what i remember, hardly ever ever go bad. take it off (w/ a flywheel puller tool) and check out the stater plate and points. they need to be very very clean and set to the proper timing. that might fix your problem.
also, use a bosch platinum 4214 sparkplug (for minarelli, not sure what for a morini).