I guess you would really need to love the old high dome poorly designed mercury in-line motors from the 60s and 70s to even want to modify one in hopes that you could make it competitive in mod outboard racing. There are just better options to choose from using OMC or Yamaha powerheads as the base for your build.
But! Some of us just like Mercury and we grew up running mercury motors...so why not.
Here are two mercury mod motors..both are creative projects.
Here is the first one...lt looks like an early 650 block with a 650 XS front end....sure starts easy!
https://www.youtube.com/watch?v=DLwm4vgH5icNow the second...this 4 cylinder has been modified to the extreme...but I can’t find any video of it running....sure looks cool!
https://www.youtube.com/watch?v=p3TGcqOa2jo