Hi Andy;
You and export the Report object source code & change the data source to the SP instead of the IM SQL syntax.
You can do this easily in PowerBuilder (IM's big brother) but IM does not expose that functionality to you in its IDE. If you have PB, you can export the object from there and then reimport it when changed. Both IM & PB use the same library concept & format.
If you do not have PB, you can expose the export/import tools via this procedure: PowerBuilder PBL compatible with Infomaker? | SCN
Note: change the tip in the link above to refer to 11.5 instead of 12.6 & that should also work for you.
HTH
Regards ... Chris