At the very least, a city business license. You might also have to register with the bureau of automotive repair, although I am not sure about that.....
Liability insurance would also be a good thing; if you hire employees, you will need to get an employer identification number for your business, and deposit payroll taxes. Workers compensation insurance is another requirement.
If you are going to use a name other than your own (i.e. a fictitious business name) then you should do a dba (doing business as) filing and publish in the local newspaper adjudicated for such publications.
If your running a vehicle, commercial vehicle insurance would be appropriate....
In addition to Mr. Hankin's excellent advice, most cities require you to have a business license whenever you are performing work in their city. Some cities even have representatives driving around looking for work being performed and issuing citations for persons that do not have city business licenses. It happens a lot to contractors. I suspect that performing motorcycle repairs would probably not draw their attention.
Also consider forming a business entity such as a corporation or limited liability company to minimize your personal liability. While it takes some money to form and maintain, it may save you lots of headache and money in the future depending on your individual circumstances.
**Nothing in this response should be construed as legal advice. The post is for general informational purposes only. This information is not intended to create, and receipt or viewing does not constitute, an attorney-client relationship.**
THIS IS NOT LEGAL ADVICE. INFORMATIONAL PURPOSES ONLY. Legal matters are like thumbprints, no two are identical. Therefore, you should discuss your question with your own attorney or if you do not have an attorney, strongly consider hiring one. The information in this posting is for general information purposes only. No part of this posting should be taken as legal advice for any individual case or situation. This information is not intended to create, and receipt or viewing of this information does not constitute, an attorney-client relationship.