The GS series of Suzuki engines from the late 70's to mid 80's are pretty much bulletproof. But the charging system is problematic. A lot has to do with the grounds that don't because of corrosion. The best article I've seen on how to test a regulator, rectifier (my GS1000 came with separate regulator and rectifier), and stator is the famous "Stator Papers" on the GS Resources site:
http://www.thegsresources.com/garage/gs_statorfacts.htm.
Rick's Motorsport Electrics, and Electrex make great electrical components. They can be had out of the Parts Unlimited book at your local shop.