The repair of aviation parts is a highly regulated industry. Your first line of action would be to contact the manufacture(s) of the flow meters to determine if they allow, and to what extent, outside facilities to repair their flow meters. Aircraft part manufactures have to get a certificate of some sort that allows them to make to components, and along with it the responsibility for providing maintenance instructions. Some companies, more all the time, do not allow outside companies to repair their products.
Assuming that you can test/repair the meter you will require the manufactures overhaul manuals for the product. It will give details for the type of test equipment required. The test equipment will have to be calibrated to a national standard periodically.
At the same time you will have to contact your aviation authority (CAAS) and determine what their requirements are. I had a quick look on their website and I believe you may have to become an approved maintenance organization. This involves writing a quality control manual explaining how you are going to meet the regulations. Regardless of what you have to do you will have to develop a quality control department and have someone who is familiar with the regulations. Often they have to take a test on the regulations. Depending on your current status regarding QC,( ie registered to another like ISO), this may require some time. You may have to get an outside consultant to help.
Good luck!