I agree that 50-60 mph on an inexspensive bike is toying with potwntial big trouble, I'd have better suspension forks and gussets at every point where one tube was welded to another, I have a huffy with a lot of milea on it and it has been a really solid bike, but its a low 30-34mph cruiser and at some point I plan to sand blast the frame and weld in some braces and I have a set of suspension forks for it, a hars quick shock on neck of that frame at high speeds could be a seriouse face plant crash......ouch!
Im curiouse about what engine you have that will be pushing he Cranny 50-60 mph....????
Low to mid 40's is plenty fast for me on a bicyle, I've seen 48mph and that was getting to the point where I felt I really didnt want to go any faster on the bicycle....if I want to safely go fast I just get on my 1584cc fuel injected Harley Davidson.... I can go from zero to 100 mph before you have time to spit and do it fairly safely..... pushing.a bicycle to 50+ mph is a risk and on a cheaper made bike its a really big risk in my opinion so please be careful bro.....