well first off the 50 - 60 mph range unless you have a HUGE area such as a 1/5 scale track will be use less with todays electric motors and batteries you can make it run a hundred if you like but the bike will be undriveable . id say if your board with 20 i would not go for much over 45 as unlike cars the bikes aquire head shake and there are many factors that make or break how fast you are able to control the bike . tires ( making sure they are balanced )
compound ( hard) = less traction (soft) = more traction the surface has alot to do with how fast you can actualy run a bike a prepaired surface ( no dust or sand ) flat with no bumps ( not the gritty worn ashphalt ) smooth is what your looking for you can run them quite hard and fast .
but alot goes into set up in order to achive this as you can throw a 7000 kv motor in it and 3 cells and haul
and get your speed BUT with out first being able to drive it at full speed with what you have you will be replacing parts more than you drive it .
lets start with batteries go lipo no lower than 25 c rating and mabe a 4000 mah pack get a lipo alarm or you can judge it as when you notice it start to slow then stop. that alone is worth a few mph and a bigger punch off the line you can play with gearing as you will then have more run time and gear it go a bit faster yet . but the goal is to keep it driveable as its easy to over power and it become no more fun . but with the brushed set up your limited to achive more youll be looking at going brushless its better in the long run if you plan on keeping the bike . report back with how the lipo worked hope this helps .