You are on the right track. The check_report method in account/wizard/account_report_common.py is inside a a models.TransientModel class named account.common.report. Then it calls print_report. In the same file, this method is not defined. But if you check say account/wizard/account_report_aged_partner_balance.py, you will see that it inherits account.common.partner.report which inherits account.common.report.

Next the print_report method returns this:

self.env['report'].with_context(landscape=True).get_action(self, 'account.report_agedpartnerbalance', data=data)self.env['report'].with_context(landscape=True).get_action(self, 'account.report_agedpartnerbalance', data=data)

Then you can check account/report/account_agent_partner_balance.py which defines a models.Abstractmodel that processes the data and renders the report thru the reder_html method. The name of the Abstractmodel is important as it should be formatted like this: report.module_name.report_name, where report_name is the name of the report defined in your xml.

So, in conclusion, at the minimum, you need to define a TransientModel class then an AbstractModel class with render_html as a method.

Hope that helps.