well they're more likely to tip over in hard braking or cornering on hills, they passed laws banning them in many places because there was sufficient evidence it was more dangerous than a 4 wheeler. i'm not saying they're terrible or anything, just what i know for sure. either way, the law was aimed at ANYONE, including eager and stupid people.
anyway, we have like 2 or 3 threads dealing with the same type of reverse trike right now.
the 10hp B&S or any utility motor will not be legal on the street, you must use a bike motor of some kind, some states allow offroad motors too, CA does not for example.
tires will need to be highway legal tires, and the smallest you'll find are 10" rim trailer wheels, but those will still be about half of the width you want. i dont know of any full DOT compliant tires that small and that wide.
you need DOT compliant tires, EPA approved engine, a quiet exhaust (measured by DMV), DOT windscreen if you will use one, brakes and lights checked by a DMV or a dealer, full bill of sale for everything, etc..
lots to do to make it road legal.
oh yea and check what license u need for it, here in cali i just need a type C license (car)