I think you need to deal with the motor first.

Check the compression,Check for spark,If it has sat around a lot you should go through the fuel system from tank to carb (dont forget the dip tube)Check all fittings,hose,connectors and clean,clean,clean.

Unless that boat is very heavy you should get at

least 40+ mph. If the Michigan wheel site is no help ask at Johns Old Mercury Web Site at:
www.pfs-ware.com Look around for a forum and excellent links.