The direct replacement for that is going to be either the Johnson Manual Pump, or the Jabsco Manual Pump. I swear I think they are made by the same manufacturer and have different stickers. Each makes a 'Compact' and a 'Comfort' model. The bowl and seat are slightly larger on the 'comfort' models, however the base and footprint should be the same.
http://www.iboats.com/Johnson-Pump-C...view_id.703188
http://www.iboats.com/Johnson-Pump-C...view_id.704323
iBoats seems to have the Jabsco replacement parts and electric flush, however I couldn't find the Jabsco Manual flush on the site. I agree that if this is just a rusted bolt issue, then simply replace the bolts with SS. I disagree with the comment on the choice steel vs. SS, only in that I know the mounting bolts were provided by the manufacturer. I had a Jabsco which used very low grade SS and they also rusted. It's called StainLESS steel, not StailPROOF steel. SS still rusts.
The issue is more than likely because Brusnwick doesn't caulk the head when mounting it, however they place the shower in the head. So guess what happens......
yep, the water intrudes and rusts the bolts and wicks into the deck where the head is mounted. When you remove the bolts, check to see if there is moisture in the bolt holes. I bet there is. Ask me how I know.