the CDI's, coils and stators are known weak links in these bikes.
Head on over to this fine forum
(specifically the SOHC Twins subforum, at the bottom of the page) and grab the factory manuals.
I'd start with resistance testing the coil, as it's the cheapest item to replace if bad. a $30 GM DIS coil will fix you up. the stock spark plug wires are molded into the coil, so you'll have to replace those too.
Then I'd do the test for the stator next, Those can be re-wound if bad.
Then do the resistance checks for the CDI. If the CDI is bad, you can take your chances with one off of e-bay (there is no NOS out there), but some guys are trying aftermarket suppliers. IgniTech out of the Czech Republic
is the most popular one right now. one guy is attempting a fuel injection conversion as well.
The Stator and the CDI are specific to the 400A, but I think the standard versions can be swapped in with some extra work.