Brian is correct.
The yamahas required a splash plate, not the merc nor OMCs. The OMCs has an option that bolted to the front of the mid section to do the same thing, had it on my 92 Johnies.
I would see if you can get a replacement splash plate from a Grady dealer and go in with flat head screws. If you can't get alum, stainelss will do, use thread sealant on screws.
The front of the motor takes a wipping of wash during certain conditions, only the newer motors have better cowl seals. Don't want the wash to enter the engine bay. The splash plate closes the gap between the bottom of drive and mid section. The plate can be cut from 3/16 in. alum plate, as well. 1/8 in. may be too thin, not sure what Springfield used.
I was surprised to see the holes were so large in diameter and as many.
On my twin bracket, there are two recesses in the casting, one for each motor.