If the Champions work well, keep them (or swap to the NGK equivalent if you consider Champion a lawnmower grade spark plug and don't want to be embarrassed if your friends stick their heads down the engine).

No need to experiment or reinvent the wheel.

Looks like the mr43t’s and BR6FS ‘s cross reference each other. Therefore, if this is true, that should be a good plug for your engine. Gap is listed as .031”. Mercruiser usual spec ‘s .035” . So a quick check/adjust gap will be necessary.