It really doesn't matter unless you want to sell the boat, or something goes boom. In that case insurance may not pay, you may get sued, etc.
IMHO, not worth the risk.

BTW, the vent screen have been tested/approvrd by coast guard. Same with starter, alternator. Same with fuel pump, carb, flame arrestor, tested & approved.
Oh, and good luck selling it. Anyone familiar with boats or a surveyor will pick those problems up right away.
Gary

I am aware of this and that some things safer than USCG approved are not condoned such as braided stainless in place of rubber USCG fuel line.

Why do you want to switch to the HEI, I have always been told the Thunderbolt is a better ignition? Just trying to learn.

Thunderbolt is not better but you also will not notice any performance gains from switching to HEI. When my Thunderbolt gave out, I switched to HEI because of the convenience

Hei has hotter spark and like jhnrckr says more available.