If you have to perform fixes to your Jawa 90 then it is always recommended to have a Workshop Manual to keep just for a reference in the event you need to know how to take off a specific component for example the Drive Shaft and match it again accordingly.
Jawa 90 Workshop Manuals gives you exact diagnostic facts with regards to your vehicle so if something is wrong it will help you figure out what the situation is and methods to resolve it. After that you can evaluate if this can be done on your own or if the job is too big it is possible to book your 90 in with a decent garage.
Jawa 90 manual