Only use OEM, WSM, Serria or EMP switchboxes..The cheap Chinesem junk on Amazom can damage your engine. I have seen several posts on other sites where cheap boxes damaged motors
Engine block is considered engine ground and lower and upper cowl and brackets are cowl grounds. They the grounding straps from brackets to cowls to engine block
All repairs start with basic troubleshooting. A 2 stoke engine must have 3 things to run. 1) Spark energy at plug to fire fuel charge under compression 2) Satisfactory compression to support combustion, a compression test will not tell you condition of cylinder! If its 10psi lower than others...
A weak impeller will cause rect/reg failures due to it overheating at slow speeds. Water does not fully fill the block cavity to reach heat sink fins. Rectifier is then damaged internally then fails.