The wheels (rims) on the Walmart bikes, especially the Crainebrook, are particularly weak, I'd upgrade to a better wheel set you're going to be powering this bike to speeds it wasn't designed for.
Also avoid coaster-brake only single speeds, try to at least have two rim brakes in either cantilever or 'V'-brake type. Disc brakes are better still.
Better the quality of the bike the longer it will last with that vibrating 66c.c. engine in the frame, Lock-Tite blue everything you might think will loosen, because they will.