I would check the battery first to make certain it is good. I would also charge it with a good smart charger & do a load test. also, make sure the battery connections are good.
Next check the fuses. Glass fuses can look good, but still be bad as a conductor can separate from an end cap, so you should test them for continuity or go ahead & get new. amazon.com might be the best place. the auto parts stores around here don't have a very good selection of glass fuses.
In the diagrams at Partzilla, the 85 EN450 only seems to have the round fuse box. it's kind of a weird. There isn't a fuse at the starter switch/solenoid. many Kawasaki models after around 1990 or so had a main blade at the starter solenoid, so I would check there as a just in case the po installed a new stater switch w blade fuse.
Fuses. If you get new fuses, you'd need a 30A AGC & two 10A AGX. AGC = 30mm or 1-1/4". AGX = 25mm or 1".
1981 KZ650-CSR. Competition, Sports & Road.