Mike, I like your old pedal grinding wheel very much. 
I would try to repair the broken stone by installing several steel staples and by filling the fracture with suitable glue with added abrasive powder.
What grit concerns, I think, that grit around 150 to 180 will work fine. You will have probably to reactivate and true the stone surface. If your stone is too coarse, you can smooth it using a stone grader.
The metal sheet between the stone and the seat served probably as a support which enabled to keep the desired bevel angle consistent along the whole blade and also to keep the angle the same for both sides.
It should not be difficult to add a water trough also. In this case the metal sheet serves as a mudguard and prevents the water spillage towards the knife grinder also.
Jan
I would try to repair the broken stone by installing several steel staples and by filling the fracture with suitable glue with added abrasive powder.
What grit concerns, I think, that grit around 150 to 180 will work fine. You will have probably to reactivate and true the stone surface. If your stone is too coarse, you can smooth it using a stone grader.
The metal sheet between the stone and the seat served probably as a support which enabled to keep the desired bevel angle consistent along the whole blade and also to keep the angle the same for both sides.
It should not be difficult to add a water trough also. In this case the metal sheet serves as a mudguard and prevents the water spillage towards the knife grinder also.
Jan