If you want to execute maintenance to your Honda SH150i then it is always a great idea to have access to a Workshop Manual to use as personal reference if perhaps you need to know the right way to remove a particular part including Gearbox and fit it again successfully.
Honda SH150i Workshop Manuals gives you particular diagnostic guidelines for the automobile so if something is faulty it can help you find out what the situation is and tips on how to repair it. You can then assess if this can be done yourself or in case the job is simply too big you can still make a booking for your SH150i in with a decent friendly mechanic.
Honda SH150i manual