the za is a very difficult engine to disassemble and rebuild correctly, most people who open them up, fuck them up. There are several parts in a za, which after 30 years are shot. The outside case bearings sit above the oil level, so if the bike sits in a garage for 20 years without being run, the bearings will pit and sieze up. The shift damper bushings also harden with age, these can be replaced with hard rubber rod, epoxy poured into chunks of hose, etc.
for these reasons, in order to kit a za, you really ought to rebuild it. if you think you are up to the challenge of doing a proper rebuild, go for it, if you just traded in your skateboard for a moped, cause its the new thing, and you want an easy slap on 40 mph, you will probably blow your shit up.
it sounds like you have a good attitude about it so far, so do a little more reading. If properly built, that 2 speed can take 10 horsepower, but if there is anything wrong inside, it will scatter and you can join the throngs of retards screaming 'dont touch it, it will break'
here's the guide i made a couple years ago, it will give you a better idea of whats involved